linny I believe it is because the file you are trying to attach exceeds the file upload limit. Unfortunately that error message does not specify that, but I think the limit is 1 or 2MB. For my screenshots, I open them up in GIMP (its in the Solus Software Center btw), and export them as a jpg, and one of the options to save is image quality, which defaults to 90% and save it that way, which gives me a file size of less than half a megabyte, so I'm able to upload it that way. Hope that helps and good luck!
