That looks like a problem with the LCD itself. I don't think you can fix the screens when they are broken (dead pixels, white screen like yours etc.) You just have to buy a new one and replace it. They can't be THAT expensive. Sorry man, but the LCD is toast.
The reason I think it is the LCD itself is because I have seen something like this before. At my local game store there was a GBA SP (same model as yours) that had on all four corners a little white mark that wouldn't go away when it was on. Next time I came in it was in the case with the ones to sell, so I asked him why he was selling it with it's screen problem and he said it was the LCD and he just replaced it and it was fine. It looked just like your picture, but yours is waaaay worse than this other gameboy. Looks like the same fix though.
Edit: You wouldn't mind if I saved that picture would you? Like I said, i've seen this a few times around the internet and once in real life so it isn't exactly an un-common thing, but yours is the more rediculously bad i've seen! xD It would be entertaining for me to have that later on down the road when someone starts complaining about a stuck pixel or two and then i'll say "you think you got it bad? look at this guy's!"