This article was previously published under Q236369
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When you lock your workstation by pressing CTRL+ALT+DELETE and then clicking Lock Computer, the previous background wallpaper (rather than the current wallpaper) may be displayed behind the Computer Locked dialog box.
This is more likely to occur if you previously selected a bitmap (.bmp) file set to tiled as your wallpaper, and you then selected an included JPEG image that is centered or stretched and requires the Active Desktop to be enabled.
The Explorer.exe shell provides additional layers that can be displayed on top of the desktop. These layers include the Start menu, desktop icons, and the Active Desktop (when it is enabled). These do not replace the desktop, but run on top of it, and can mask the images that are set to be used on the desktop itself. When a computer is locked, it is taken out of the user's context and the Explorer.exe shell is cleared, along with its layers, revealing the images that are set on the desktop underneath. The issue described in this article is most noticeable when a .bmp image is selected and applied as the desktop wallpaper first, and then a JPEG image is selected for the wallpaper. The effect is compounded if the Picture Display settings are changed when you apply the JPEG image (for example, changing from Tile to Stretch). This causes the small tileable .bmp file that was previously displayed to be stretched to fit the screen dimensions when the computer is locked.
To prevent this behavior from occurring, clear the previous wallpaper by clicking None in the Select a background picture or HTML document as wallpaper box, and then clicking Apply. Then, select the Active Desktop image you want to use. When the computer is locked, the computer will display the background color and no previous images.