Device: Dell Chromebook

Issue: I've resized the image- but it won't use as profile picture. The image is a .png and is 300x300, but it won't show.

Error Message: It was not possible to determine the dimensions of the image. Please verify that the URL you entered is correct.
The submitted avatar is 0 pixels wide and 0 pixels high. Avatars must be at least 260 pixels wide and 260 pixels high, but no larger than 2600 pixels wide and 3000 pixels high.

IMG:
/images/upload/7c7qjk8o63xd.webp

It works perfectly for me, there must be something wrong with your original image. Make sure you are sending the good file (it can't see it as an image). Either re-download it from your post, or re-save it with your image editor