Best thing would be take the image you are attempting to load, use a graphics program like Gimp or whatever, and crop it to 120x120 pixels. Save this as a new file, better as a png for smaller sizes, you can try jpg as well. Then attempt to upload this. Chances are your image exceeds the pixel size, or the file size is too large.
I’ve just done this twice now. First image was 600x600 pixels, size of 269kb. After selecting the image, wait for the uploading to finish. Only then will the button be accessible to save the avatar. However, you have to wait before clicking the button to save - since the image needs to load and show against the text “custom picture”. Then when you see the image has loaded, click save changes. The second image was 385x385 pixels, size of 332kb.
So it is possible to upload an image successfully.