If you want to use a certain pic as wallpaper, you may need to resize it. Use one of the online resizers - click here: BlackBerry Cool xxx187; Blackberry Wallpaper Generator
It will allow you to upload your pic, let you check off what BB series phone you have and then it will automatically fit the picture in the proper dimensions. It'll ask for your BB email and then email you with a link to the resized picture. You just click the link, and then save the pic then select pic as your wallpaper.
Don't worry, it's super easy and asks you all the questions. I do this with all my pics and works like a charm!