Here's a great example of how hard it can be to define disinformation in practical terms. I've seen this photo show up across multiple Facebook feeds, getting huge engagement. Everyone loves it. And yet, the picture doesn't show what it claims to show

It says the soldier isn't carrying the donkey because he likes donkeys, but because he doesn't want the donkey to trip the mines in the field. But this is a well-known war photo, and it shows exactly what the text says it doesn't — a guy who took pity on a donkey

The man in the photo is a member of a French Foreign Legion militia unit made up of Muslims (the Harka) who found a young donkey lonely and hungry during an operation in the field, and took it back to French Foreign Legion base, where it became their mascot

The photo and its context is mentioned here (warning, page is really long and takes awhile to load): http://foreignlegion.info/history/13dble/ 

But the alleged lesson from the photo seems to resonate with people so much that they feel compelled to share it. It's not literally true, but that doesn't matter — it says something that *feels* true to them and that's more important