The only Photo CD I have is a copy I made of a friend of mines Photo CD a couple of years ago. At that time I did not know that Photo CD's has some sort of copy protection (according to Ted's Unofficial Kodak Photo CD Homepage), so I copied the CD without thinking that much about it. So I really don't know if my copy of the Photo CD is actually a "Photo CD" if you look at it from the standards point of view.
As the Photo CD specification is based on the CD-i Bridge specification to allow for the playback of Photo CD discs on CD-i players (accoording to the CD-i FAQ), and the United 3155 with VCD/SVCD capabilities is supposed to play CD-i discs, it should be possible to play Photo CD discs.
However, when I tried my copied Photo CD in the United 3155 with VCD/SVCD capabilities, it did not show anything on the TV. If this is because of the fact that my Photo CD is copied or that the player is not fully CD-i compatible, I don't know.
The player did however do something when I tried the Photo CD in it. It was possible to press "NEXT" and "PREV" on the remote control, and the "Chapter/Track" indicator in the display on the player stepped forward/backward.
If I can find an original Photo CD, I will try with that one. Then we will know for sure.