Hey, sorry to bump this thread, but Windows 8.1 actually
does use a static image for its start button. Well, sort of.
The images for the start button are located in
C:\Windows\System32\twinui.dll They are actually PNG files (rather than bitmaps like in Windows 7). Here's what they look like:
Start Button image with large taskbar icons...
...and small taskbar icons.
So as you can see, they are nothing more than regular images. But the start button changes when you hover over it. This is where things get a little more complicated. They are still images, but they are partially animated by the Metro UI. Here are the hover images
Large taskbar icons
Small taskbar icons
As you can see, they're pretty much just black boxes with the Windows 8 logo cut out. Metro will fill in the cut out logo area with whatever color you have selected on the start screen, and also create a shine/gloss animation over the logo.
So there you have it! Hopefully this has been helpful to those who need it. And for those who want it, here they are as a Classic Shell start buttons:
Large taskbar icons
Small taskbar icons