Answer
Check the j2ee application server logs first, since they often contain more specific error detail about why a particular image failed to load than what's visible from the client side. For non-logotype images specifically (distinct from the standard company logotype mechanism, which has its own dedicated setup), there aren't many formal rules to follow — except that on Apps8, these images specifically need to be placed at the report_designer_graphics\<module_name>\<report_id>\ path on the server for the runtime to actually find and render them. A missing or incorrectly-placed image file at that exact path (rather than image size) is the most likely reason a non-logotype image like a footer graphic fails to render even though the report otherwise generates correctly.