As I understand the strictest interpretation of the rules, a photo makes it an ad.  An ad doesn't need to have a pic, but since guys are visual creatures, it's probably a smart thing to do.  A provider may include her link into any (or every) reply that is on topic.

As with most things, the devil is in the details.  Are the picture threads ads?  Well, they certainly are marketing, but since everyone has a fair shot and they're kept to once or so per week, the benefits to the guys seem to outweigh the negative (making this just an ad board), so I look the other way towards them.  If a lady repeatedly throws a "me too" post on every topic to get her website out there, that would be frowned upon.

I hope that helps.