If I'm understanding you, you have 20 images and you want to place one of the images placed on each of the 15x20 pages, right?
On the Parent page for the document (assuming all pages share the same parent), create a graphic placeholder frame. With the parent placeholder selected, choose Object > Fitting > Frame Fitting Options to choose the way you want the images to be fit on your document pages)
Select the 20 files in the order you want them in page sequence (the order you select determines the order place). Click the placement cursor on the placeholder frame of each page to place the 20 images
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