Well, first would be to make sure that:
- The file size in pixels is the exact one the site demands.
- The file format JPG or PNG
If that is ok, and what happens is that your file is too big still, then I would recommend lowering the quality of the image. I do this for the webcomic I work for, but try to check it slowly:
Your image quality would start at 100%, then you'll lower it bit by bit once you achieve your desired file size. Usually, you can do this without having to go lower than 80% so good luck!