There's a little globe icon added in the bottom-right corner of external links, so you know that tapping the thumbnail will open a link rather than an image preview, but in its current form it's hard to see against darker thumbnails.
The posts I've highlighted with arrows in the image are both links, but it's almost impossible to tell.
Yeah, a confirmation pop-up (or at least the option for one) when you tap on a link would go a long way to helping with that.