Jazz Jackrabbit 2 Online

A pair of AngelScript libraries for manipulating and creating sprites. Each library file includes its own documentation, so open up either one in a text editor to see the list of functions/classes/etc.

TrueColor lets you draw (and resize, rotate, etc.) full 32-bit (including alpha channel) palette-independent images.
Resize takes existing images—in jjPIXELMAP, jjANIMFRAME, or TrueColor::Bitmap formats— and shrinks or upscales them by arbitrary amounts, with several different resizing algorithms included.

(Discuss in this JCF thread.)