Music in the Public Domain covers most anything that the copyright
owner has been dead for 75 years or more. That means all the
classical music is in public domain. The actual recording is not,
but you are treading on much less dangerous ground with classical
music. Canon in D is a very popular classical piece for wedding
music.
Most photographers own the copyright to their images. Most
musicians who have been signed by a record label do not. The
musicians who are not signed, and there are MANY who are very
talented, are eager to have their music heard. Find someone locally
or nationally but unsigned, and ask them if you can use their music
as background for your slide show in exchange for a link and music
credit. Anything you've heard on commercial radio stations has been
signed by a label and the label owns the copyrights. Labels are
next to impossible to deal with unless you are Pepsi. The musicians
have no say once they sell their rights.