''incompatible jpg''

I took the Kingston memory card out of my CanonA720IS digital and moved my pictures to IPhoto on my Mac so that I could edit them. When I finished editing, I used the same Promaster card reader/writer to return the pics to the memory card, since hooking up my camera to my tv is the only way I can show large pics to my entire family at once. But when I checked them in the camera, about 20% showed only a thumbnail with the message ''incompatible jpg,'' or had just a question mark where the image should have been. Ideas?