I can see the issue on my iPhone 4S running iOS 6.1.3
, it does disappear when I zoom in. But I can also reproduce the issue using Google Chrome Version 26.0.1410.64 m
. When I zoom in to anywhere above 110%
(obviously it is easier to spot at even higher zoom levels) I can see a green line appear for just a little while, which then disappears as the image becomes sharper within a few seconds. At 300%
the line doesn't disappear anymore, it will always be visible, and at the same time the image does not become sharper anymore, but then at even higher zoom levels, the line is never there.
Seeing the image becoming sharper and then seeing the line disappear is leading me to believe that the problem might be with the image being saved as an interlaced image. Obviously you might have done this on purpose, as it is not really a bad thing. And in that case you would also know that interlacing an image enables it to be shown as a degraded copy of itself while it has only partially been received by its intended recipient (in this case a visitor on your website).
Now I have never heard of any issues with interlaced images in mobile safari, but doing a Google search did turn up some results. Including a result to a question with some useful answers here on SO. Problems seem to have started with the iOS 6 update. Most problems seem to be fixed by either turning off transparency on the image or by saving the image with the interlacing option turned off.
So I'd suggest you try those options and see if that fixes it. Or if you really need interlacing, maybe use a different image format which supports interlacing (browsers may handle it differently between formats?).
What I find quite interesting is that it seems like Google Chrome doesn't use interlacing on higher zoom levels, I guess it doesn't make sense to try to get the image to be even sharper when you're zoomed in that much already?
I would find it interesting to read some documentation on how different browsers handle interlacing for the different formats which support it. I couldn't really find any (but I didn't have a very good look). It might shed some light on why some issues have started to appear in iOS 6 and higher, to me it looks like mobile safari on iOS 6 does not support interlacing PNG images at all, and since I could reproduce the issue in Chrome, I thought that it might be an issue with WebKit based browsers across the board, but my Safari 5.1.7
does do the right thing on all zoom levels.
Anyway, I hope one of the suggestions will solve your problem.