Rodrigo,

From the form designer you would select the imagelist control and click the right-mouse button and select properties. Select the Color tab and you should see an entry in the Properties list called MaskColor, use the Color Palette list to select the colour that will become transparent.

If you want to do this programmatically then their are two properties called MaskColor and UseMaskColor. MaskColor can be set using the RGB() function and UseMaskColor is a boolean that tells the imagelist wether or not images have a mask.

HTH
Neil