>I need to create a 450*450 pixel transparent GIF and add some small images on this. I have found functions in GDI+ to create and transform BMP to GIF, but I don't figure out how to make it transparent. A sample would be very helpful.

The creation of a transparent GIF requires that you select a color to act as the transparent color. So, I am not sure if there could be ways to automate that. Whenever I do a transparent GIF, I am selecting a transparent color based on what I see as the background color on the image, of what could be used as the background color. This is usually a color that is not found elsewhere in the image. When there isn't such a context, I would simply drop a color in the desired area and would later select it as the transparent color.