I had the same problem. The pictures I used for my profile picture were fine, but once I tried uploading them to any threads, they were too big. If you go into the thread that talks about how to upload pictures, it should have a download link for a program called Photofiltre. Download and use that to resize your pictures to about 300pixels (that's usually a good size, but not always). In the top right it will tell you how many "kb" your picture is when you adjust it. I think the thread will tell you "your picture needs to be 100kb or under" when you try to upload it. Stick to whatever "kb" it tells you to and then save your picture. When you try to upload it again, it should work.
