To scale and center selected layers to fit the width or height of the composition frame, while preserving the aspect ratio of the layer, choose Layer > Transform > Fit To Comp Width, or Layer > Transform > Fit To Comp Height.
See After Effects Help:
I'm curious why you're using an unconventional composition size like 720x1080. The standard for 720p is 720x1280 and the standard for 1080p is 1920x1080. If you're going to share this video online it might be worth it to consider choosing a standard comp size.
Having said that, you won't be able to fit a square photo or an almost square photo at 4000x4000 into a standard sized comp without cropping it. Even a 3:2 comp, which is much more square, will still crop your photo on the edges. Even Todd's suggestion will crop your photos:
Here's how it looks with Layer > Transform > Fit To Comp Width selected:
Here's how it looks with Layer > Transform > Fit To Comp Height selected:
If you don't care about fitting your images into a standard sized comp you can simply drag your photos onto the Create a New Composition icon at the bottom of the project panel to create a new comp with the same settings as the footage you're dragging into it:
That will bring up the following box where it will ask you whethere or not you want to create a single composition or multiple compositions, which photo to use for the dimensions, how long you want your still duration to be and whether or not you want to sequence the layers and create an overlap with a dissolve. Creating an overlap with a cross dissolve might make it extremely easy for you to make a slide show of your photos. I would start by trying these settings and then adjust accordingly:
Finally you can go to File > Scripts > Scale Composition. Then change either the height or the width to scale the composition and all of its contents:
I think I will just have to make a larger standard composition size. So if I work in 4,000 x 4,000p for a project, will it render (through AE then Media Encoder) in 1080p with good, high quality? I don't want to use the cross-dissolve for transitions, I'll probably add transitions in manually when I use them. The slideshow is intended to have minimal effects.
I'm a beginner in video productions, here at Uni. The professors don't know much past the basics of the software.
If you want your final result to be 1920x1080 (True HD), then the final comp you render should be 1920x1080. If your pictures are never going to be larger than 1080 tall, then I would suggest resizing them in Photoshop (using the sampling type best suited for downsizing). If you have a lot of pictures to do this to, you can set up an action and run it as a batch on a whole folder of images (creating a new folder with your edited images). That way you don't have to worry about any scaling in After Effects.
Szalam is correct. 1920x1080 is true HD, which is probably all you need for your purposes. Also, downsizing your images to 1080 pixels tall in Photoshop will make your After Effects project run faster and it will allow you scale all of your images the same if they're different sizes. What manual transitions were you thinking of adding?
My apologizes for the week-late reply. Starting htis new semester has been busy for me.
To Szalam: How could I go about setting up an action to automatically downsize/rescale my photos? This is interesting.
BenMarkus, what is your recommended method of sizing down in Photoshop? And to answer your question, I had the mindset of:
(photo)>(photo)>*quick flash*(photo)>(photo)>(photo)>(photo)>*fade in, slow music*(photo)>etc
Well I didn't know exactly what to search. Now I do, and I thank you.