There are different rules on attribution for images and other digital content depending on where you obtain them. Be sure to read and understand the copyright and/or creative commons attribution requirements. Some sites don't require attribution.
Not sure about using an image?
Take a look at these sites:
ccMixter - Thousands of free sounds and music to download and use.
Piano Society - Classical piano music uploaded by users that is free to use.
YouTube Audio Library - Download background music for your video, completely free.
Free Music Archive - Many genres available as well as curated sounds to help you find what you need.
SoundBible - Free sound clips, sound bites and sound effects.
Audionautix by Jason Shaw - Original, electronic songs that are free to use.
Partners in Rhyme - Some pay and some free content. Also has effects, Midi and loops to use.
Moby Gratis - Created by the artist, Moby, Online application system, which registers you to use the site.
Incompetech - Background music to download by Kevin MacLeod.
Sound Search Engine - Huge site for finding sound effects.
Classic Cat - Find classical music for video soundtracks.
Purple Planet - Lots of free full length songs to download and use.
Zap Splat - Free sound effects with a huge variety to choose from.
Cylinder Preservation and Digitization Project - Mostly Public Domain songs that have been digitized before 1924.
Freesound.org - Samples and search engine with a forum for discussions.