George,
There are upwards of about 12,500 OP varieties, and I don't know if you're asking about OP's ( open pollinated or hybrids or both.
If you're new to this fun of growing tomatoes then for some of the best pictures I suggest you go to tomatogrowers.com ( OP's and hybrids)as well as Marianna's ( I forgot the link, but you can find it at Google, as well as Baker Creek which is rareseeds.com Also with some good pictures and descriptions is Victory Seeds. Also check out seedsavers.org for about 40 varieties with pictures in the public catalog. SSE members have another several thousand to chose from.
At the top of the this website there is a list of seed sources, which includes all of the above, and you might want to check out heirloomtomatoes.com and Sandhill Preservation as well. Neither one has pictures and the descriptions of the latter are minimal, but Sandhill has the lowest prices around and gives lots of seeds and has excellent quality since they sell no seeds older than 2 yo and that's quite a feat for about 400 varieties listed.
And that should give you enough to look at for a while.
