I was using a program called 'Restorator 2007' (which has a free trial for 30 days) and I discovered that the start menu image has a default height.
Click to view attachment^ A prime example (if this is against the law (posting microsoft images) let me know or if your a mod delete it!)
There are 6 images you can try modding.
Click to view attachmentAfter you mod the images height wise, you can go ahead and download
Resource Hacker.
If you haven't done so already, you need to mod your uxtheme.dll file!
Also, swith your current Windows theme to Windows Classic (anything that isn't a vista theme anyway)
Open Reshack as administrator and click open. Migrate to: \WINDOWS\Resources\Themes\Aero\ and open aero.msstyles (to open that file you need to switch the little drop down next to the file name input to 'All Files'.
On the left hand side, there is a menu. Click 'IMAGES'. Start with scrolling down to 706 and when you click it, another item should open up with the number 1033. Right click 1033, click replace resource. Select your image.
Resource type: IMAGE
Resoruce Name: 706
Resource Language: 1033
Do that for all the files replace 706 with their numbers (708, 709, 715, 717, 718) and those are the only images as far as I know of.
Use your keyboard and hit Ctrl+S when you are done. (an automatic backup is made of the aero as aero_original.msstyles if your theme blanks out on you or is errored up (which it shouldn't)).
Good luck.