Merge pull request #2231 from Carreau/ja Improve Image format validation and add html width,height This addresses the first two bullets of #2053 Validates first argument to constrain format to png, jpeg, Lets user specify width, height, or both and emits appropriate html I introduced a couple of class constants to minimize the number of times I saw 'jpeg' and 'png' and document the _ACCEPTABLE_FORMATS. I added one real jpeg image to the test directory and one zero-length gif file to survive the file not found error in super() Closes #2053 Add `assert_is_none`, `assert_is_not_none` to `nose.tools`