I have started to link to images on DBpedia and if I pull any in because the links are unstable then I add the image metadata in the RDF. It is my understanding that thumbnails representations 128x128 or 135x95 or smaller are ok to use, based on cases involving Google and Bings use of thumbnails.
thumbs: probably generally ok. In the case of dbpedia (which is the interface to Wikipedias) the images are however truly open content and usable in full resolution, given they are attributed appropriately and the license is cited.
It would be useful to create an open but attributed set of images for each species that we could share. It is my understanding that if they are open, Amazon might be willing to host them for free.
Any repository that provides open content licensed images is a great contribution. Images can be put on commons.wikimedia, but cannot be semantically annotated very well there. Species-ID/OpenMedia does allow that (running Semantic Mediawiki) -- we welcome collaboration to improve semantic annotation.
Images can also be put on Morphbank, but care has to be taken to choose an open content license. Most contributors choose to submit the images only under the Closed Content, almost non re-usable "non-commercial" clause (which does not only exclude those making profits, but also non-profits requiring cost re-imbursement as well as anyone gaining non-monetary advantages from using the image).
Gregor