The filename of the picture displayed over the button surface when the button is disabled.
Supported bitmap formats are BMP, ICO, GIF, JPG, TIF and PNG.
To make the picture background transparent use the PictureTranspMode property.
To render a picture color transparent use the PictureTranspColor property.
In order to perform changes to Pictures and Animations settings at Design-time take a look to the How to display bitmaps and animations section.