Update Image class to support absolute sizes.

[mchippingtonderrick-afl]

Supporting absolute size values can be helpful for scenarios where specific sizing is required, and especially where the sizes of the images being used may be variable.

(I know this is an old repo, and there are far more up to date Python 3 versions, but unfortunately there's some software which still only allows Python 2.7, so there's the chance this might help someone else that's encountered the same issue.)