Just in case you haven't seen it all yet, my MarioSonicGames.com site includes a huge collection of free Super Mario Brothers and Sonic the Hedgehog (and other related Nintendo and Sega) goodies for your enjoyment, in addition to all the free Flash and Java online games and Flash movies.
There are loads of Mario and Sonic images, animated GIFs/sprites, MP3 and MIDI music samples and WAV sound effect samples, all of which come from the all-time classic 16-bit era of both game franchises. There are also dozens of desktop wallpapers/background pictures, plus lots of other Nintendo and Sega surprises for you to find. Here is a list of links to all the relevant pages...
Mario, Sonic, Nintendo and Sega images, animated GIFs/sprites, music and sound effects (plus Flash and Java games and Flash movies):
Super Mario Bros.
Sonic the Hedgehog
Mario and Sonic Bonuses, More Mario and Sonic Bonuses
Extra Mario and Sonic Bonuses, Even More Mario Bonuses
More Games-Related Images and Music, Even More Games-Related Images
Mario, Sonic, Nintendo and Sega desktop wallpapers/background pictures:
Super Mario Bros.
Sonic the Hedgehog
Super Smash Bros.
Donkey Kong
Pokemon
Megaman
Super Monkey Ball
Puyo Pop Fever