You should be able to use your image editing software to make them match pretty close.
Current sizes:
4runner - 213x100
corolla - 133x100
If you resized the 4 runner to 1/2 height, it would be about the same size (106x50).
You could crop the corolla's top/bottom of the pic to get it to a simiilar height so they match . Either case in my current browser the images are next to the mpg pic .
Something that hasn't been mentioned here; When I added the code, the preview was just empty. If you don't have any data added to the fuel log, the signature won't display. I put some bogus numbers in there (since I'm just getting started) and it worked like a charm.