Does anybody else have the same problem with making new club banners, which all come out with a blurry left corner?

It's not just one single banner for one fan club which didn't come out the way it should be, but *all* of the club banners that I try to make never seem to come out the way I intented. So, does anybody know what I am doing wrong? Is it a matter of pixels, meaning the picture resolution, o is it a domanda of size, o both? All the banners that I make are the same: with a blurry left corner. I had to stretch the picture to make it fit, and I had to resize it to fit, but somehow, I always fail to make it right. Let me rephrase my original question: How to make a perfect club banner? I really need your help on this one, and some consigli on how to make a club banner of the right shape, size, even the right resolution this time. Please help, if te can. Thank te for your help.