Usually the browsers display images with the default screen resolution which is 96dpi. In this way if the image has a greater resolution, e.g. 300dpi as it is in your case, then its width and height become larger 300 / 96 = 3.125 times than the actual size. I believe that this is the problem and to fix it you just have to use images with 96dpi.
Hope this helps.
the Telerik team