1) It is common courtesy to cite your source. After all, thats where the photo/drawing came from, the author or poster may have spent time and money finding it, scanning it, or whatever. It is only polite to give them credit for the post.
2) Sources help us to understand the picture's context. A drawing can be a real manufacturer's drawing, a drawing done by an artist for a magazine, or a doodle someone knocked out of MS Paint last week. If we know a drawing was published in Aviation Week at the time and is captioned as a manufacturer's drawing, we can be pretty certain its accurate.
3) Sources help authors and researchers track down material for their books. If you post a picture from Aviation Week and don't tell me where its from, I can't go and read the article that accompanied it. I may miss an important fact. If you tell me the issue, I can follow that lead myself.
4) Sources help people find interesting books or magazines to buy, or websites to visit.
If we can just follow these rules and put our sources, it makes life better for everyone.