On the native app, by clicking on an image of the Activity Stream, this image is displayed in full screen (popup).
However, I noticed that with this full screen display, the zoom and sharpness level is very low.
While on the internet browser (Safari) and the full screen display of images (popup), you can zoom a lot and see the details in the image.
The resolution of 1280px allows enough zoom.
The difference between the two displays is very clear.
On the native app, the image is blurry.
Here is a video capture that compares the two versions :
And a image comparison :