Fix image download test to not rely on assets outside the codebase

We should probably have a proper test image eventually, so that
we can use e.g. image dimensions to double-check the results, but
this way at least our tests won't get broken by unrelated changes
in other repos.

Change-Id: Iffffd94fde99c53da59217a5181ad0560fce314c
Merge "Fix image download test to not rely on assets outside the codebase"