You probably don't want downsampling as it will definitely blur the image. However, if your image has way more in it than you care to have in your avatar, I would suggest cropping it. The displayed avatar is a square so you could at least crop it into a square (unless it already is one). If it is still too large, you will have to compress it. If you don't have software on your computer that can do this (or you don't have access to a computer), you can use websites like this. That particular site is for PNGs (which I believe are most common for screenshots). If your image is not a PNG, there are some links on that page to other compression sites as well as format conversions sites.