Wirelessly posted (TP2: Opera/9.80 (Windows Mobile; WCE; Opera Mobi/WMD-50369; U; en) Presto/2.4.13 Version/10.00)
Are you saying that your background on the Sense tabs is darker than it is as a picture? I believe that is by design because of the white text. There is no overlay that I know of. It actually makes the picture partially transparent to darken it.
See if
HDWall can give you want you want. It gives you greater control over the background in exchange for a little more work. I use it for all of my backgrounds.