[This is preliminary documentation and subject to change.]
If this bit is set the picture on the button or a bitmap or an icon control is displayed in its "natural size". If the picture is smaller than the control, it is centered, if the picture is larger, it is chopped in case of bitmaps but it is shrunk in case of icons. If the bit is not set, the picture is stretched to fit the button or control rectangle.
On CheckBoxes, PushButtons, and RadioButtonGroups this bit has any effect only if the Bitmap or the Icon is set.
On Icon controls and buttons associated with icons this bit is ignored if the IconSize bits are not set!
Bitmap
CheckBox
Icon
PushButton
RadioButtonGroup
None.
0x00100000 = 1048576