David Dorward wrote:
Does anyone know of a tool like this? Or of any libraries (Perl modules by preference) that would make it easy to write it?
The latter. Many years ago I wrote some thumbnailing stuff that solved the aspect ratio bit (but not the bounding box interactive cropping bit) in perl using imagemagick and/or gd whilst at high school. There was a good o'reilly book, "Programming web graphics with Perl and GNU software".