Yo use dynamic background.
TWG checks the resolution of the browser and then resizes your background image that it covers all the background without loosing the image ratio.
in your case the image is 180x74. If TWG detects e.g. a height of 740 then it resizes the image internally to 1800x740 and then only the left part of the background is shown.
use background images which have the same ratio as normal browser windows and where it is no problem if not all of it is seen.
And the image is not resited 100%. I alwas do this in 100 px steps because this image is cached and you don't want 500 background cache images
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- Michael