My 2 cents:
I love mercurial, especially because TortoiseHg offers a very nice GUI for it. I have no experience how it works over networks, but it should perform reasonably well, given the fact that for example the Mozilla project uses it.
Both mercurial and git are suited for large projects (=many contributors, many files). Git is used to develop the linux kernel after all. However, both mercurial and git are distributed version control systems, so both, by definition, suck for repositories containing many large binary files, especially if those binary files change frequently. Storing many large binary files and modify them every now and then is exactly what we will be doing with the girl images if we store their tags as XMP metadata.
What we should do, IMHO, is keep the source code in mercurial (or git) and move the ressources somewhere else. Options:
a) SVN. Subversion can handle large binary files with frequent changes somewhat decently, but we will still see a significant increase in repository size over time. However, due to its centralized nature, the growing repository will not bother developers, only the server hosting the repository.
b) Dropbox, Mega, ...; No version control, therefore no repository size increase, but also no incremental updates.