By default, the TransparentColorTo property is -1, which indicates that the control's TransparentColorTo property defines the color to be shown as transparent on the current picture. The Opaque property indicates if the picture is shown as opaque or transparent.

The TransparentColorFrom / TransparentColorTo properties have effect it:

• Opaque property is False ( by default )
• picture's attribute does not include the PICTURE_TRANSPARENT flag ( for instance a PNG picture with transparency, includes the PICTURE_TRANSPARENT flag )
• TransparentColorFrom / TransparentColorTo properties points to valid colors ( different than -1 value ). For instance, if one property is defined and the other is -1, the first one defines the transparent pixels, while if both are specified and points to value different than -1, any pixel between them is considered as transparent.  

If The TransparentColorFrom / TransparentColorTo properties have effect, any picture where these apply defines the pixels as:

• any pixel with a color between TransparentColorFrom and TransparentColorTo is defined as transparent
• any other pixel that's not transparent is opaque.

The control supports almost all type of pictures like

• PNG (Portable Network Graphics) is a raster graphics file format that supports lossless data compression. PNG was created as an improved, non-patented replacement for Graphics Interchange Format (GIF), and is the most used lossless image compression format on the Internet
• BMP file format, also known as bitmap image file or device independent bitmap (DIB) file format or simply a bitmap, is a raster graphics image file format used to store bitmap digital images, independently of the display device (such as a graphics adapter)
• JPEG file format (seen most often with the .jpg extension) is a commonly used method of lossy compression for digital images, particularly for those images produced by digital photography.
• GIF ( Graphics Interchange Format ) is a bitmap image format that was introduced by CompuServe in 1987 and has since come into widespread usage on the World Wide Web due to its wide support and portability.
• TIFF (Tagged Image File Format) is a computer file format for storing raster graphics images, popular among graphic artists, the publishing industry, and both amateur and professional photographers in general.

If using the PNG format, the control handles automatically its transparency / alpha blending ( if saved with transparency ), unless the Opaque property is True, so in this case, any TransparentColorFrom or TransparentColorTo property has no effect. 

For any other picture type, you can use any of the following to define the transparent region of the picture:

• TransparentColorFrom, specifies the transparent color to define transparency part of the current picture (to). 
• TransparentColorTo, specifies the transparent color to define transparency part of the current picture (to).