If you take a 800 x 600 image and turn it into a 800 x800 you either have to have a border on the small side to make it 800, or you have to crop the image to 600 x 600 and then enlarge to 800 x 800. What is your intent?
A square format is more unusual than a rectangle. Very few pictures are printed that way, probably for aesthetics.
I'd be happy with a white background. So if the image is 200 x 600 then the image becomes 600x600 and the 200 side had 200 each side of it white.
Some of my images are small or large so my concern is making them all a set size and it distorting small images or squishing big images when it wa snice to have the large images also.
In an action, you can make the background a layer, duplicate the layer, rotate 90 degrees (with the center transform proxy active), and then reveal all. Delete the duplicate layer and size to 600 pixels. If you save as a flat image format, you can have PS add white borders. Otherwise add it yourself in the action. If you're looking to process images with less than 600 in either dimension, I think you'll need a script to do what you want.
Mass resizing tutorial
So you're wanting uniform size? You can use Image Processor script along with an action to make them all square in 1 simple click of the Run button (after you apply the right settings, of course.) Yes, all however many thousand images you have! We want to get this done in 1 swoop, which is possible, especially since all the images are ending up being the same size. I'm going to hold anyone's hand and do this step by step for anyone who has minimal to expert experience in photoshop. This explanation is very simple to follow.
1: Preparation to make them square
Heres the first step for the canvas size to be made uniform and square-y with a simple action. If you don't want square-y but still want uniform, skip this step. Let's say you want to make those thousands of pictures all 600 x 600 pixels. This is the preparation step. Make a new blank document 400 x 400 pixels. Make a new action, and name it Canvas Resize, and hit the record buton to start recording your actions. Change the canvas size to 600 x 600 pixels, and after you do that, stop the action recording. Close the document.
2: Organize so PhotoShop can process all your photos in 1 swoop
So we want to make everything 600 x 600. I'm using CS4, but CS3 and CS2 (and probably older versions of PS) all have a feature called Image Processor. This is the time to consolidate all the images you want to process into 1 main folder for an easy 1 click operation. Make two folders on your desktop and label one Pictures and the other Processed. You can either toss all your picture files which you want to process into the Pictures folder for a giant mess, or if you're organized, toss a bunch of organized folders into the Pictures folder, and you can keep your folder organization automatically. I'd recomming copying (NOT moving your pictures, COPYING) and organizing your pictures to be processed in the Pictures folder so that you don't potentially ruin your current organization of your photo archive. I've made my messes before moving files around and trying to figure out where I pulled them from.
3: Now that we're organized:
After you've consolidated your files, we will use photoshops Image Processor feature. First, click on File -> Go to Scripts -> Click Image Processor. A dialogue box will pop up with a bunch of options. Section 1 (section numbers are on the left) is "Select Images To Process." Click the Select Folder button and find the Pictures folder on the desktop with your images in it, and select the folder. Click OK. If your images are in multiple sub-folders inside the Pictures folder, and you want to process everything in each folder, check the Include Sub-Folders box so itll process everything inside. In section 2 of the Image Processor, "Select the Location to Save Processed Images," select where your batch of images is going to be saved. Select the Processed folder for this. If you have your photos organized in a sub-folder folder structure, and want to keep that organization, check the "Keep folder structure" box so it will structure your folders inside the Processed folder the same way it was structured inside the Pictures folder. Now for the file type. Choose whichever file type you want to save your photos as. Check "Resize to fit", and in the resolutions for width and height, put the same number in both. This will NOT stretch the images, but it will take whichever dimension is larger and scale it your specified number, and keep the aspect ratio the same! For example, say you have an 1800 x 1200 image, which is a 3:2 aspect ratio. If you type 1200 x 1200 in the dimensions, wether vertical or horizontal, it will resize the long dimension of the image to 1200, and the shorter to 800 and keep the 3:2 ratio. Cool, huh? This makes it painfully easy to do thousands of pictures overnight if you want them the same maximum dimension wether vertical or horizontal.
Now for the magic:
In section 4, "Preferences" in the Image Processor, you can have PS apply actions to pictures after they've been resized. Ain't that fancy? Check the Run Action box, and select the Canvas Resize action we created earlier. Since we created an action that changes the canvas size to 600 x 600 pixels, the action of making it square will be done all in 1 shot! Any picture 600 x ? will be made into 600 x 600 with whatever color you chose your background to be in recording your action. Now, click the Run button, and your computer will angrily churn all your images into perfect 600 x 600 pixel squares. All numbers I used were for expample only. Pick whatever number you want. If you want 3000 x 3000 squares, punch in 3000 x 3000, make the canvas resize action for it, and go for it!
Anyone who reads this and tries it, let me know how easy or difficult it was to use this tutorial for you. This method is how I organize the CDs or DVDs which I submit to register my photos to the US Copyright office. I generally make a batch of 800PX JPEGs, images out of all the photos I selected to copyright, burn em, and I'm done within the hour. For my first copyright CD, I made a copyright CD for registration in less than an hour for 4 years of my photographic work.
And P.S.: Upsizing always dulls or pixelates a photo unless you're going to take the time and enlarge each one with enlarging software, and that's a whole new batch of processing in its own. And.. yes, I wrote this whole tutorial just for this post, lol. I enjoy teaching technical knowledge. Hope this helps and whoever reads this enjoys it!