When dealing with more complex image transformations, I usually cheat and construct the image using Inkscape. This gives me an SVG file which I then turn into a template. From these templates I produce more SVG files, which Inkscape then renders into PNG bitmaps.

This is of course not really fit for running on a server, as firing up the Inkscape process for each image is likely a bit costly, but for my small needs, it provides a quick turnaround.

Update: Looking at the Inkscape website, I find UniConvertor, which seems to be Inkscape without the UI.