Also looking for this kind of functionality for the Yammer connector for a while! Take a look at the similar/overlapping idea that I've posted two and a half years ago called "Post on Yammer with image attachments".
That's great. I recommend everyone here to vote for @hvandekamp post with 59 votes
Yes. It is really limiting to not be able to pass images through Flow into Yammer!! PLEASE IMPLEMENT! Vote it up here: "Post on Yammer with image attachments".
I agree, this is a necessary feature to implement.
@geommm wrote:It is necessary to be able to include at least a link and a picture when posting to Yammer. Posting simple text messages is not of much use. HTML tags would be nice if they can be implemented.