Is there a way to import photos into a project, rather than just a video.
Also, though this is a different subject, when I import an MP3 into a new project, the sound is distorted. However, when I play that same sound on the same computer--but not in Sonar--it sounds fine.

For the first question, no. Sonar basically allows you to use a video as a time reference.
For instance, you are creating a musical score for an action scene.
You can create and manipulate audio, and add it to the video, but you cannot manipulate the video itself.
You need to use a video editor to get your still photos into a time based format.
I believe Windows Movie Maker can work with stills (not sure because I haven't worked with it for a long time).
The more elaborate programs definitely can.

The second question is probably a matter of having Sonar use a driver mode that is appropriate for your sound card.
The basic media players are much less demanding of the sound card, usually playback only at very high latency.
Selecting MME, and setting the buffer size close to Safe is probably your best bet.