A graphics library for quick creation of PNG or JPEG images
http://www.libgd.org/Main_Page

The gd graphics library allows your code to quickly draw images
complete with lines, arcs, text, multiple colors, cut and paste from
other images, and flood fills, and to write out the result as a PNG or
JPEG file. This is particularly useful in Web applications, where PNG
and JPEG are two of the formats accepted for inline images by most
browsers. Note that gd is not a paint program.