craftgrrl | Either I'm blind or it's not in FAQ?memories

I just remembered I've got quite a few bathtiles lying around (white/cream). They're dirty, but otherwise in good shape and have never been used (got them for next to nothing at a yard sale, they were left over from re-doing their bathroom). They're square tiles and aprox. 4"*4". What I was thinking was to paint them and put felt under them to make coasters out of them. I think it was on some DYI/decorating show on BBC-America a long time ago. However.... I can't remember what type of paint they used so it doesn't just come off again every time you put something on it, and I've no clue what to use as a sealant to prevent the image from getting damaged with use.
Do any of you have a clue??? I'd love to do this (even think I could get hubby to join in on that one!) at some point this year, so any suggestions would be appreciated.
Many thanks!