When you hold your finger down on a post in the iOS mobile app, then move your finger, the page scrolls, no matter how long it is until you start moving your finger - this is fine.
When viewing posts with images in, you can tap on the image to open it up full screen for zooming etc - still fine.
When you hold your finger down on an image and begin moving it fairly soon, the page scrolls - also still fine.
When you hold your finger down on an image, but pause because you were just finishing reading a sentence, then begin scrolling, the image gets the "active" overlay and when you finish scrolling and release your finger, it opens up full screen - rather annoying and breaks the flow of reading the post.
I think the act of beginning the scroll action should cancel the "I want to open this image for full viewing" action.
iOS App, Version 1.4.0
Update: I just realised that links exhibit the expected behaviour too - holding down then pulling a link to scroll won't navigate to the link upon releasing your finger - it is just the images.
As mentioned in the comments, using the website in Safari on iOS displays the expected behaviour - holding your finger on an image makes it gain the "active" overlay, but as soon as you start scrolling it loses that overlay, and doesn't open when you release your finger.