For path pictures, look through Robbin's Basic Pathology or, if you have it, Robbin's Pathological Basis of Disease
For MRIs, I used HY Neuro. I only had MRIs of the brain on my exam, so I guess I got lucky by only reading HY Neuro. Has anyone else been given a question dealing with an MRI from another part of the body??
For histo stuff, I think first aid is reasonable. Questions involving actual histology images (of normal tissue, not diseased tissue) are so low yield it's funny. Questions involving histology images of diseased tissue are more abundant, but robbin's basic pathology is sufficient.
I'm not sure of a good source for CTs, but I used Moore's Clinically Oriented Anatomy and Netter's Atlas for the cross sectional drawings, and then just converted the drawings into CTs in my mind.
And don't forget that for all the above images, you should be able to convert each one into a drawing or diagram. Sometimes rather than give you a simple abdominal CT, they'll draw one out. Or they'll draw a heart with coronary vessels labeled and so forth. There aren't a lot of ways to study for drawings, you just have to make some educated inferences from imaging you're familiar with.