That’s expected behaviour as the Tweet can only have one “attachment” that renders, which is what the DM URL counts as. We’ve started to make this a bit more clear in the new documentation relating to the upcoming changes to Tweets, where we’ve mentioned a forthcoming attachment URL parameter. Unfortunately you won’t be able to do this (add an image and have the DM prompt appear) at the current time, and I’m not aware of plans for this to change, but I’ll pass this feedback along.