At my last internship we had been using garage band to create music. I know I have a list of royalty free sites as well somewhere, I'll have to try and find them.
Not sure if you know the rules regarding copyright on using music. If your slideshow is in the public realm (on a website or blog) then you must deal with copyright. EVERY piece of recorded music, no matter how old, is covered by copyright, unless the author has expressly stated that the music is royalty free. As suggested, you can use the tracks in Garage Band to create your own music. But if you aren't doing that, try a Google search for "royalty free music" you can find sources that will allow you to use music without fees, many only requiring giving them credit for the music in your show. I've also had success in working with local (or not so local) bands who allow use of their music for credit as well.