GT Metrix punishes me with an "F" for my pictures, because the avatars, which the users upload, don't all have 40x40, but different sizes. The reason for the downranking is the rendering time to scale these images to 40x40. The more users there are, the longer the rendering time will probably be.
Is there a way to force users to only upload images in the correct format? Or to automatically rescale them correctly while uploading them?
What is the "correct" format and size? Is there a standard?
GT Metrix is just a consulting site, nothing else. Not an authority of any kind.