You are correct and if you set the background through the Today app in the settings panel, it will have (I think) a 65% transparency, and it is not adjustable from this setting. However, if you open the image for viewing from the Pictures and Videos app (just click on your image or open it from the programs list) and then click MENU and "Set as today background", you can choose the transparency level (the lower the number the more to actual it is, 0 being no transparency).

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-Michael
Need help setting up your i730/i830 or other WM2003/WM5 Phone, as well as the other hacks I have done, check out the MRailing Guides and Links