A number of Ernst Haeckel's images have been released by Scott Draves as PNG files. Mr. Draves is releasing the images under a creative commons license; here are the details: "The alpha (transparency) channels in these PNG files are released under the attribution share-alike Creative Commons license. The original bits are from Ernst Haeckel."
I've reposted one of the images here. There are a ton more, and they're beautiful.
As a side note, kudos to Mr. Draves for having a mouse prominently pictured on his homepage. Three cheers for rodent fans!