Have you just downloaded Firefox emblems as images?
Not sure you can change the start page image per se.
However, if you like the images you have downloaded why don't you try out
a whole new browser theme and that'll rock your world.
Seriously though, check out a full theme
Click the Firefox button (top left of browser)
Click add ons
Thousands of full themes in there - try some of em out!
Does anyone know if you CAN just change the Firefox logo on
the Google homepage which is what the OP wants I think - got me