If you put a frame on the master page, it will appear on all pages based on that master.
For that master graphic frame, select it and choose Object > Fitting > Frame Fitting Options. This sets the defaults for how pictures will fit when you place them.
I don't know if this works on a pc, but on a mac you can select the whole list of images and copy, then when you paste into excel you will get a file list.
You can use this list with data merge to create your whole 300-page document, with fitting options and all with a couple of clicks.
there are several ways of doing this.
1. You could use a script.
2. You could prepare a document
3. You could do it while placing
Let's begin with #3.
If you loaded an image into the placegun, do not click.
Just drag out a rectangle with the mouse. Do not let go until the you reached the right size.
While you are at it, not let go, you could use the arrow keys to change position.
On to #2, prepare a document:
Add a new document with the number of pages you need from the start, say 300 with "no facing pages".
Go to Master A page and draw out a graphic frame exactly the size of your page.
Select the graphic frame, go to the frame fitting options that control how a placed image will go into the graphic frame.
From my German InDesign (I hope you can identify your analogue English controls):
1. Frame fitting would be: Fit Proportional
2. Best chose the center dot of the 9 proxy controls (or experiment)
Go back to your page 1, don't do that on Master A page!
Load all 300 images into the placegun.
It would require only one click per page to get all the 300 images in.
#1 write a script that will place all 300 images.
Or ask a scripter.
Kasyan Servetsky has written a very great script to do this!
If the images have "repetitive" names, as: Image_001.jpg, Image_002.jpg, … It's very simple and quick to play the game!
Just need an automunbering para style!
Thank you very much