Nitpick: non-software can't be "open source," as the "source" refers to source code. Neither does "open source" refer to libre usage terms, as it only means the source is available, not that it's licensed favorably.
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.
