As i'm fairly newbie using XP (and WB4 under XP LoL) and still running WIN98 in dual boot (with WB 3.51 to be able to get an explorer background image), i quiet always change (or add) an explorer.bmp bitmap to the skins and so, i also change the background window color - called "Window" in the [Colours] section - (to fit with the explorer.bmp bitmap LoL).
So just with doing those two things, the same skin lose the benefit of viewing the large shellstyles pictures to be displayed under XP, and i was wondering why ... (and perhaps why also Stardock doesn't want to bring back the opportunity to get explorer background images in WB4 ...). It has nothing to do with the changing or adding off an explorer background as i thought firstly, but with the changing of the background window color : to be displayed, the large ShellStyles pictures HAVE to be their backgrounds color EXACTLY the same as the background color !!!
Just an example : if background window color is defined as 255 255 255 - as it is in a lot of skins, the shellstyle large picture have to be drawned on a 255 255 255 background. Though, if you just change background window color to 250 250 250, the shelstyle large picture is no more displayed (or u have to change his background color also to 255 255 255)

I don't really know if it would be possible, but i wonder if there would be a new feature with skinstudio that would automatically change the shellstyle large background image color the same way background window color is changed ??

Second observation : i did notice that if these ShellStyles large Bitmap are done in 256 colours, they are not displayed as well (at least if using the video card in 32 bits), and to be displayed i had to convert them to 16 millions colours ... (tip for skinners who draw them in 256 colors p )