This maybe the case in Western world , but I think there are much more possibilities than you can think.
Many people here ( I mean in the Eastern ) use forums not just a communication and debate tool , but also for sharing pictures ( something like 9gag ) with their own opinions , and even use as a minor private goods trading platform (imagine a yahoo auction or ebay without any charges , just for friends to trade and sell their beloved stuff).
For examples of the further use of a forum , you can have a look of how a Chinese made forum called Discuz . ( Now this forum is dominating the whole Chinese community , and I do hope Discourse and go and challenge them )
It’s true that they added too many rubbish features that made the forum bulky recently, but my main point is : “Forum” as a forum is only valid, when they can attract more people and gather at the same place for discussion, and thumbnails is one of the ways to achieve this in my opinion.
PS. My recent experience tells me , my readers are unwilling to change to use a new forum (Keep on using the Facebook group) , even if I promoted them to use a such revolutionary Discourse forum to them.