We are aware of this issue . Let me explain in some details. There are two facts that need to be taken into consideration. First - zooming resizes pictures as well and second - certain percentages of zooming do not resize the picture enough so you get this appearance. For instance if the picture for the left corner is 5 pixels wide and if you resize it to 170%, that would be 8.5 pixels. Unfortunately, browsers can not resize the picture to that type of fractions yet, so the resize would either 8 or 9 pixels. In one case, you will have a perfect look, in the other, you'll have one additional pixel.
I guess we could alter the skin and do a thing or two to fix all controls and skins, but it will take quite a lot of time to go through all possible variations. However we have noted it for consideration.
the Telerik team
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