I wish you could do wallpaper, but you're just limited to the start up screen (which displays for two seconds), and the power off screen (which I couldn't get to work with my picture).
Here is the procedure I found on the Toyota Entune Forums:
Here's how to change the startup image on the entune nav:
Load a usb device, such as a flash drive with the image of your choice. The images file name, including the file extension, can be up to 32 characters. Name the image without spaces or strange characters. When saving the images to a USB, name the folder that the startup image is saved to “StartupImage” and name the folder that the screen off image is saved to “DisplayOffImage”. If these folder names are not used, the navigation system cannot download the images. (The folder names are case sensitive.)
The images file extension is JPG or JPEG. Image files of 10MB or less can be saved.
Up to 3 images can be downloaded. Plug the flash drive into your car usb port.
On the entune nav system:
Press the “SETUP” button
Touch “General” on the “Setup” screen.
Touch “Customize Startup Image”
Touch “Transfer”
Touch “Yes”
You can also delete and switch between screens.
You should now see your new startup screen flash by when the entune system is started each time.