The wallpaper choosing dialog has changed a little bit in Fedora 18 (GNOME 3.6). However, when choosing a wallpaper, I get three "tabs" at the top of the "Select Background" dialog that give me the option to choose between Wallpapers (the system wide stock ones), Pictures (GNOME searches for these pictures in the "Pictures" folder in your home directory), and Colors (block colours and gradients).
So if you put the Photos that you want as the wallpapers in your Pictures folder, they should appear under the "Pictures" tab in this dialog:

UPDATE
It also seems that if you have pictures in sub-directories, GNOME will not show them in the Pictures tab. This is a bug that has been previously filed in the GNOME bugzilla: https://bugzilla.gnome.org/show_bug.cgi?id=682126
Also, GNOME will not show images that have been taken with the gnome screenshot tool. ( I filed a bug for this -- https://bugzilla.gnome.org/show_bug.cgi?id=693228 ), but this is intended behaviour for GNOME, so it is now closed NOTABUG.
If you really want a screenshot as a background, you can workaround this by opening your image in the GIMP and exporting it back to the same file with "File > Export to yourfilename.png" in the GIMP. This will trick GNOME into showing the file in the Pictures tab.