I tried to trim it in "paint"
Use the Image-->Stretch/Skew to "stretch" it to a lower size-numbers lower than 100 will make it smaller, as it's a percentage.
This will lower the quality of the image, but without knowing what size image you're trying to use in the first place, and therefore how much it needs to be shrunk, it's the best I can suggest.
The other possibility is that the file size is too large-in which case I would suggest converting it from bmp (which it probably is) to either .jpg or .png. Paint can do the former, although I've had issues with its conversion, and Irfanview can do the former, the latter, and more (plus, it's free).
I'm actually not positive if the avatar function supports .bmp as I never tested that, but it's something to look at. You don't specify what extension you're using.
