Thanks for your reply but I have not explained it correctly. I tried to use a jpg file from my computer as my avatar. The file name appears as though it has downloaded but when I go back to my control panel some time later to check, it has disappeared!
There's a free image editor called GIMP for Windows, Linux and MacOS that makes it easy to adjust your file size and resolution. Just open the image, select all, go to image, scale image, and then choose the resolution you want. Then file, export as a JPEG and set the compression level around 80. That should make it small enough.