The generate versus store thumbnail depends on the thumbs themself, are they likely to change regularly? if so generating them would be a better option. But if the are always the same with very rare alterations then storing them would be better.
It also depends on the kit that you are using. A nice high powered server with a monster storage would then come down to personal preference. The person viewing the page will probably have to pull the images down the wire if the images are stored or generated.
Basically if your server has a large storage space I would store them instead of generating each time, a jpg thumbnail only takes a couple of k storage. but if the storeage is very limited then generate them.
hope that helps
Mark.
