Michau, thanks for asking. It is unlikely something is in the public domain unless it was created before 1927. I don' tthink they had bikinis then
And yes a model release as well as the photographers releases are required to be properly licensed and these must be obtained in writing.
Buyer beware. Although it is possible to get RF (royalty free) images for under $10 each, their use is severely limited. I think that
photos.com offers the best terms, but read the fine print. Even at photos.com you "may not redistribute or sell the content" and you are never allowed to use RF images as content on a website, e-greetings, or send-a-friend pages. these uses are in essense distributions.