I believe it´s the images placed in the
content and
inactive modules areas.
And yeah, it sucks when you have a good image that doesn´t have the required size and most of the times the simplest thing to do is to search for better ones. Or you can go ninja and do something with the image involving tedious cropping and stuff..
I can´t add much more since I´m not really good at it but try checking out profiles from members that have won Potm recently (or a bunch of times) to give you a general idea of what makes a great profile. And try do keep it simple.
Also, check
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for tips and I believe dimensions are displayed.