The Background menu on the 720 will allow you to change the image shown during boot-up (currently the iFly logo). To do so, you'll need to save the image as an 800x480 pixel, 24-bit, BMP file. Then copy that file onto your SD card in the root directory (not in any folders) and rename it "LOGO.BMP". Then you can eject the SD card from your computer, put it back in your 720, and open the Setup Menu. Touch the 4th menu item down on the left, the wrench and screwdriver, then the BackGround tab across the top. Press the Change button, then OK. Once that's done saving, you can reboot the unit and see the new image.
It can also be done from a 740/740b, with a different method. You can use most BMP images so you don't need to resize or rename. Copy the files onto your SD card then put it into your unit. Boot it up and touch Multimedia, then Photo. The Photo Viewer will open with buttons across the bottom. From left to right, they are Play Slideshow, Slide Time, Booting Image, Rotate, Playlist, and a display of what image of how many in the folder. Touch the Playlist button and choose SDMMCDisk, then your image. If it opens correctly in the viewer, touch the Booting Image button with the screen and up-arrow and wait a few seconds for it to save. Reboot the unit to test.