Correct use of the image_exists function

Since image_exists is a function we don't need to
wrap it in []'s when testing that is passes. Doing
so actually means it will always render true.

Do this instead:

 if image_exists && [ -n "$USE_CACHE" ]; then

Change-Id: I9cbb5e74a038b157c75d1c91bef086a620b1cca6