Ah, I did miss the first post, I assumed that $25 had been offered,
The licence agreement on the site allows any and all commercial use as long as credit is given , no payment or other reciprocation is required.
The validity of the license on shipbucket images is also questionable since it is granted by a 3rd party site and not the creator/owner of the content, but that's a trivial debate for another day...
All original rights still maintained by the original artists, and that includes the right to sell their work. By submitting it to an upload session, they are allowing Shipbucket to host a copy of their work under the fair use agreement. Also, the credit requirements can be interpreted as stringent enough that maintaining proper credit in a non-digital medium is not easy - much easier for the using organization to go to the original artist and get whatever is needed from them.