[swiatek]

in my honest opinion ->

for windows users, *.zip will do just fine... why use any other method of compression??
if you have linux users prawling through your community then go with the *.tar.gz

For media files, I just use them in whatever files they come in. No need to bother to convert file types. Idealy, .mov would be best because it's WMP format. everyone just loves Microsoft.

Photos-depends. Using PNGs you can squeeze lower file sizes (only on smaller images) than JPGs. Note: IE does not support transparent PNGs. I've never been a fan of GIFs: image quality is never good enough for HQ photos / PNGs typically create smaller file sizes than GIFs when producing small images/icons.

To each his own.