Once in a while I have issues in posting JPG/PNG images edited on the Paint tool (Windows), but I never noticed on this forum ( ok, I admit that I do not upload pictures so often ). Right now I could not inform precisely how this happens, but the board system informs that it is at the wrong format; for instance I try to post a picture saved as PNG, but the system informs that the JPG format is incorrect.
Therefore, just as an attempt, next time you could try changing its format to another and check if this works.