Recently and finally upgraded from X1 to Professional...    Can a Sonar project be used to set music to a photo slide show? 

The easiest route I would suggest is to offload the music track from SONAR and do the mating in Windows Movie Maker. Movie Maker is pretty simple to insert images in the timeline that can be transitioned like a slideshow.

If the slide show is already made (in a video editing program), you can use Sonar to compose music to it. If the music already exists, then as mettelus says use a video editing program to make a slide show set to that music.

Windows movie maker is actually a pretty good tool.  Here are a couple of 'shows' i made for someone dear to me.
 
https://www.dropbox.com/s/lury6u7t8ck4zpp/wou.wmv?dl=0
https://www.dropbox.com/s/cvd4xwzlpe2o0cz/6week_update.wmv?dl=0
 
The first one is more sync'd to the music.  (music edited in Sonar - not mine).  The second one is not as sync'd to the images.  (music production all made within Sonar).
 
All the panning, swipe, zooms etc., are all part of movie maker.  Guess I'm just trying to show what can be done with the aforementioned tools.   Your mileage may vary 
 
If you have access to the slides that make up the show, then this is a very viable option IMO.  It does take a little bit of time to get things synced up (fades, swipes, etc.) but the end result is pretty good.
 
Good Luck!
 
<edited for typos>