Uploading picture needs improvement.

I'm not sure why this happens,or how to fix it. When I upload a picture I've made in the Gimp, only 1/4 of the picture (the very top) is uploaded. The rest of the picture is grayed out. I have the picture scaled to the precise size that is required by the website, yet this keeps happening over and over. I had to redo the current one 4 times in the Gimp and ended up taking the middle part of the picture and putting it at the top of the picture. I'm also using Ubuntu,but I don't see why that would make a difference. Any thoughts?