Also make sure you give your images helpful names. I believe the preferred way to do this is with dashes. So if you were selling professional Nike footballs, to use sequencehosting's example, it would be good to name the image
professional-nike-football.jpg.
Google does now
support underscores as word separators, so that would probably be OK too.
And think about the keywords for which you want to rank. It's pretty unlikely that your hypothetical web shop would rank for
football, for example, and even if it did, it's likely that most of the traffic would be low quality: people searching for
football are probably more interested in news or results or their own team. Whereas people searching for
nike football are much more likely to be shopping.
So using the phrase
Nike football, as opposed to just
football, as part of the alt and title tags and filename makes it more likely you will rank for that phrase and should get you better quality traffic too.