Even if you don’t have Photoshop, but you’re running Windows, you can do this the old-timey style. Hit up Start > Run… > mspaint. Now, keep in mind this won’t be quite as fancy as Photoshop. File > Open… your image and use the Select tool to chose the area you want as a background. You’ll notice little squares all around the selected area – you can drag those around to resize, and the dimensions of the area will show in the bottom-right as you move around. Get it to your device’s screen size, but just make sure to not let go of your mouse button, as the more you warp the image and let go of the mouse button, the more distorted it gets. Edit > Undo is your friend in that case. The best way to go is to shrink from the sides rather than the diagonals, since it’s easier to make sure you’ve got the width or height right, rather than both at once.
After you’re satisfied with the size, Edit > Copy your selection, then File > New for a fresh file. Go to Image > Attributes… to change the canvas size to your screen size, then Edit > Paste your picture in. Now you can File > Save your picture as a JPEG and get it onto your BlackBerry via USB or e-mail.