Similar post to the one from a few months ago, but maybe providing some more detail. Basically the images cover the content but not the viewport. I've tested 3 devices:
1. Safari and Chrome on an iPhone 5S running iOS7;
2. Safari on an iPad running iOS6;
3. Safari on an iPhone running iOS6.
The issue is the dynamic address and navigation bars at the top and bottom of the screen that disappear on scrolling on both the iPhones I tested.
In example 1 running Safari, on page load both bars are present at the top and bottom of the screen. When scrolling down, both bars slide off screen, the image stays the same size but produces a letter box at the bottom of the screen (slightly higher than the bottom navigation bar).
In example 1 running Chrome, there is only 1 bar at the top of the screen so when scrolling down the picture itself marginally resizes (zooms in) to cover the whole screen. When you scroll back up, the bar reappears and the picture goes back to original size.
In example 2, Safari looks like a desktop browser with a static combined nav / address bar and the plugin works fine.
Example 3 only loses the top address bar with the bottom navigation staying put. The end result, though, it the same as in example 1 with the letter box at the bottom.
Interestingly, it's also only a problem in portrait screen orientation, not landscape. Just going to test this now with a different orientation image.