You can set and change your user pic both for your personal account and public profile in the same way.
1. Please hover your mouse over your current picture. A camera sign should appear, please click on it once.
2. A pop-up window will appear and offer you to attach a new picture. Please note that you can drag and drop a required file, as well as upload it via the menu. The size of the file can't be more than 5mb.
3. If the picture is too big, you will be able to choose what part of it will be visible by moving the direction marker. You can also change the size of the visible part by clicking on one of the 4 squares on the sides of the visible part.
4. Once everything looks like you want, press Submit and your picture will be updated.