Check to make sure the file size is below 5mb. Last time I talked with @Wander on this, it was sorta agreed 5mb was a good file size to accept since most images out their are below that size.
Jpg and png files will upload just fine.
Gifs will upload but they get converted to an MP4. I’m not sure about the exact file size with it exactly it probably is at that same standard to require the gif be below 5mb.