The additional .css you see is loaded from a resource file by Sitefinity and it cannot be stopped. However, when you restyle some of its styling (body tag or RadMenu elements for example) the generated resource .css get overriden and your changes are applied. This works perfectly well for images, just make sure that your folder structure is like the one here:
Also, make sure that the links to the images are like the following:
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I'm glad your problem was solved, can we close the support ticket now?