You could script this, but for just a single go, I think it would be easier to do this semi-manually, roughly as described in Scott’s comment to the question. Since you say you already have all the images ready and numbered, it shouldn’t take too long, even if it will be a fair amount of repeated clicking. Shouldn’t take more than about five minutes, I’d say.
Note: I’m assuming here that you have all your numbered images in the same folder. If you don’t, start out by moving them all into one folder.
Here are some (simplified) steps to get it going:
Create a new object style; under Frame Fitting Options, tick
☑️ Auto-Fit and set Content Fitting to
Fit Contents Proportionally
Add empty frames in the location where you want the images on all the parent (master) pages that are in use in the document; make sure they’re all located in the same place on each page and have the dimensions you want, and have the object style from step 1 applied (might be a good idea to place these frames on a separate layer, just to make it easier to isolate them later on)
Once you have the frames all set up, go to the first page in the document where you want to insert the images; make sure you’re in Normal Mode, not Preview Mode (so you can see the borders of the empty frames)
D to place images; select all the 500 images in the Finder/Explorer window that’s opened and hit Open
Click inside the prepared parent-page frame to insert the first image (you don’t have to override it first)
Page Down to jump to the next spread, then click in the two prepared parent-page frames on the spread – repeat until all images are placed or you’ve run out of pages in the book
That should give you the result you’re after.