Straight from MSDN
http://msdn2.microsoft.com/en-us/library/ms648045.aspx
fuLoad
[in] This parameter can be one or more of the following values.
LR_LOADTRANSPARENT
Retrieves the color value of the first pixel in the image and replaces the corresponding entry in the color table with the default window color (COLOR_WINDOW). All pixels in the image that use that entry become the default window color. This value applies only to images that have corresponding color tables.
Do not use this option if you are loading a bitmap with a color depth greater than 8bpp.
If fuLoad includes both the LR_LOADTRANSPARENT and LR_LOADMAP3DCOLORS values, LRLOADTRANSPARENT takes precedence. However, the color table entry is replaced with COLOR_3DFACE rather than COLOR_WINDOW.
code:
LR_LOADTRANSPARENT 0x0020
/ 
