Certain revision properties are automatically attached to revisions by Subversion's commit process, and they carry information about the revision.
These photos can be quite large, so as is common with sites of this nature, you want to provide smaller thumbnail images to your site visitors.
Now, you can get this functionality using traditional files. You can also store your captions and datestamps in a similar fashion, again separated from the original image file.
Otherwise, you run the risk that a future release of Subversion will grow support for a feature or behavior driven by a property of the same name but with perhaps an entirely different interpretation. Just as files and directories may have arbitrary property names and values attached to them, each revision as a whole may have arbitrary properties attached to it.
The same constraints apply—human-readable names and anything-you-want binary values.
You can modify, commit, and revert property changes as easily as you can file content changes.
And the sending and receiving of property changes occurs as part of your typical commit and update operations—you don't have to change your basic processes to accommodate them. While there are only a handful of such properties in use today, you should avoid creating custom properties for your own needs whose names begin with this prefix.
And Subversion does, in fact, use properties—both the versioned and unversioned variety.
Certain versioned properties have special meaning or effects when found on files and directories, or they house a particular bit of information about the revisions on which they are found.
But it certainly leaves much to be desired in terms of both performance and likelihood of success, especially if the scope of your search would require a working copy from the root of your repository.