Take a look at the Biodiversity Heritage Library’s Flickr page:
You can also look through their own webpage directly:
The Biodiversity Heritage Library improves research methodology by collaboratively making biodiversity literature openly available to the world as part of a global biodiversity community.
They have a massive library of images, most, if not all, of which are public domain now. The Flickr page is for images (they’re taking a break in uploading), and their own page includes images, books, field notes, papers, etc.
The image pages on the Flickr page also link to the text the images came from, which can, in most cases, also be downloaded in its entirety. Everything is properly documented as well.
Many of the items in BHL’s collection are in the public domain and free to reuse without risk of copyright infringement. Check the field to determine the copyright status of any given item in our collection.
BHL does our best to indicate the copyright status of each item digitized by our partner institutions. Ultimately, BHL does not hold copyright on the materials in our collection and cannot grant permission. It is up to you to review the copyright status of the image or item you wish to reuse and abide by any copyright restrictions that may apply.
The images tend to be JPGs, but it’s easy to turn them into PNGs. Most image editing software will have a tool to intelligently remove the background if you’re wanting an image that’s just the organism with the former background invisible.
Different image sizes on Flickr can be gotten to by clicking the “Download” icon in the lower right of the image pane, then selecting the image size, or, better, going to the “All Sizes” option and selecting from there.