"open source" definitely refers to libre usage terms:

http://opensource.org/docs/definition.php

The "source" for a bitmap image (PNG/BMP etc) can be a vector image (SVG, PostScript etc) or maybe a multi-layer bitmap image (Gimp, Photoshop etc). The "source" in "open source" refers to that the files that were transformed into the files in question are available under a libre license, not what kind of file format or language they are represented in.