DPI is irrelevant inside AE. It is only relevant when going to or coming from the paper-based world: scanning (from inches to pixels) or printing (pixels to inches).
I agree with Nick — don’t scan your images bigger (pixel X pixel) than they need to be. If the images will never be bigger than full frame, they don’t need to be bigger than 1920×1080.
Whoever scanned them should do the math next time: (paper size) X (scanning DPI) = (digitized pixel size). So an 8×10 at 600 dpi becomes a 4800×6000 image, which is way too big for a 1920×1080 (or 1280×720) HD frame, unless you were planning to zoom into (about) a quarter of the image. Use a lower DPI, one derived from your quick calculations. The extra time taken doing that will pay off in a much faster AE workflow.
To solve this and speed up your workflow (on this job), either shrink the images in PS to a size closer to your actual needs (as Nick wrote), or look up “Proxies” in the Help.