The SPA525G can display any size JPG image, but the phone's screen is 320x240 pixels in size.
The JPG images can have up to 32-bits of color depth.
Basically the above means that you can display any JPG image that you like. [Please remember that this is a phone, so don't expect it to deal with megapixel-sized files. A photograph or a logo will be under 200kB in size.]
Configure Wallpaper Selection Using the phone:
Copy your favorite photos on the USB key into the \pictures directory.
Insert the USB key into the SPA525G's USB slot.
Press the setup button [looks like a page with top-right corner bent]
Press 6, User Preferences
Press 3, Screen Preferences
Scroll down to Wallpaper
Navigate right to display the Select an image screen This causes the phone to check the USB key's \pictures directory
Press View display a full size version of the image Use the Previous, Next, and Back soft keys to scroll between the images
Press Select to choose the new background image
Press Set to save your selection
Press the setup button [looks like a page with top-right corner bent] to exit and display the phone's regular display with the new wallpaper