Thumbnails Resolution on Web Sites

I know must of you have your own website, I´m trying to make one of my own and need to figure it out what should be the optimun resolution for an inch long thumbnail.. Because is so small I´m trying to keep the most quality possible but need the page to upload in a quickly manner.. So.. what you use? 72 ppi? less? more? (I´m thinking in a 36 thumbnails gallery for each page).

I also heard that there are programs to do this automatically (I´m doing it on photoshop), so If anyone knows about it...