It might be good to use the thumbnail generating class at http://phpthumb.sourceforge.net/ as the thumbnail processor available. It has a number of features that would be nice to enable via parameters or wrapper processors:

1. A border (configurable color & width) can be applied.
2. Thumbnails can be a fixed dimension regardless of source aspect ratio and background filled with configurable color.
3. Border corners can optionally be rounded (independant horizontal & vertical radius)