Setting an arbitrary photo app for the Dash shortcut is not possible in Ubuntu 11.04 but I think it will be possible in 11.10 which will be released in October.
In 11.04, Ubuntu uses the following list in order to fill in the View Photos shortcut.
- Shotwell
- F-Spot
- gthumb
- gwenview
- eog
It will pick the first installed application from this list so if you want F-Spot instead just uninstall Shotwell (and of course, make sure F-Spot is installed). Unfortunately, there is no way to set the shortcut to an app not on this list (without recompiling the source code yourself which is not recommended).