I've been using a Windows Phone 7.5 device (in my case, the Nokia Lumia 800) for a few months now, and while I found a few apps to create a custom lock-screen instead of using an existing photo, none did have the graphics effects I was looking for. Looking on the intertubes, I found a Photoshop template by AJ Troxell which put me on the right track (but I obviously wanted to use Corel PHOTO-PAINT). Combining some elements and ideas from AJ's template with the actual tiles layout of the phone, I created a Windows Phone 7.5 lock screen template in Corel PHOTO-PAINT X6 that has the exact position of the tiles, and with which I could clip-mask images within the tiles.

Using the actual PHOTO-PAINT X6 file, you can customize the artwork, text and colors / gradient easily to make it your own. For example, to replace the artwork, drag&drop the new image and re-position it. Then, in the Objects docker, select the different tiles you would like to constraint the image within (they are all in the squares group) by CTRL-Click (I picked the first 6 in this case as I have a gradient at the bottom that goes to 100%). Then go to "Mask > Create > Mask from Object(s)" in the menu or simply press CTRL+M. Now select the image in the Object Docker, right click on it in the docker, and select "Create Clip Mask > From Mask". Playing with the layers in the Object Manager, you can make sure the new image is below the Squares group and you are good to go.

Final comment. You might be able to open this PHOTO-PAINT file in an older version, but some of the things won't display correctly (the gradient in the lower tiles for example).

Happy PHOTO-PAINTing!