BAD WOLF and Stryker -- if you want people to be able to find a topic (thread) you have started when searching via hashtags, then yes, add the hashtags in the first post (I like them at the end of the post, but I suppose anywhere would work). And people should know to follow through the rest of the topic (thread) in case there are more images, info, discussion etc.
If you really want a specific post other than the first post to also pop up in hashtag searches, I suppose you can put tags in it too, but that might be too much work; also, if there is too much of that, it might flood the search results and make that more unmanageable.
If you really want a specific post other than the first post to also pop up in hashtag searches, I suppose you can put tags in it too, but that might be too much work; also, if there is too much of that, it might flood the search results and make that more unmanageable.