Its an old rule with forums, the idea behind it is to keep the image size to a minimum so people browsing the site on any internet connection can load multiple images in fast time, not affecting their experience. (say you're loading 20 -50 profile pictures/page, maybe more, with no limit at 10mb each, things start to add up (for something as unimportant as a profile picture), especially with slow broadband speeds or mobile. Now given, maybe it is a little outdated with faster and faster broadband speeds nowadays and 3G/4G too, but it still stands.
Try saving your images to PNG file types from JPEG or whatever else first. Then if needs be, crop it down after that.
Any questions, ask away!
Chris