Hit up File > New to get your your blank file started. Now open up your chosen background separately through File > Open… . This can be the original file, or a screencapture (you can copy whatever is on your screen by hitting the Print Screen key on your keyboard and paste it into a blank file).
If you did a screenshot, Crop your image first, then Select > All , Edit > Copy and Edit > Paste it into the new image. Now Edit > Free Transform to begin resizing. If you want to stay true to the original, hit the lock button between W and H in order to maintain proportions. This may also involve some background work to fill the background in. In this case, we’ll take the Eyedropper tool to get the image’s background colour, and then use the Paint Bucket tool to fill in the white space. Otherwise, you can just rough the image in to fit.
If there’s any extra work you want to do on the image (redeye, blur, lens flare, whatever floats your Photoshop boat), now is the time to do it. After you’re all done, hit File > Save as… (or Save for Web… if you’re concerned about space) and save the final product. Throw it onto your BlackBerry either by dragging and dropping it via USB while your Media Card is in Mass Storage mode, or e-mail it to your BlackBerry’s e-mail address.
Here are some examples we created using this technique and our t-shirt design.