If you already figured that out, but still couldn’t upload, that happens to me sometimes (usually if I try uploading a high-resolution image). Sometimes if I take a screenshot of the image, that lowers the resolution enough that the image can upload.
Not sure if any of this addresses your issue, but… here’s hoping!
If you do mean the character sheet avatar, then when you're editing the sheet, there is a "Change Avatar" button on the top right:
Edit: to use it in your posts, you need to have GM approve your sheet, then you'll have a "Post As" option above the field where you're writing your post.
Also, clearing cache can be found in browser settings, usually. On PC you can also try Ctrl + F5, should work for most browsers.
I think you have successfully changed it you may need to refresh your browser for it to show up (hard refresh not soft refresh eg F5 or look it up for you phone)