How to do that? very easy.
-You will need a picture of yourself This is me.

-Next you choose a costume and download on your computer. This is the one I have chosen.

-Open them both in your workspace in PhotoImpact.

-Make your costume image, RGB data.

-Select your picture and paste in your costume image, send to back.  You may have to resize down your photo to fit the area for the face, if you do, while it is select, do a focus on it to make it clearer and get rid of some blur.

-Now that your face is in position, while it is select, erase the unwanted part around it. Don't forget to trim.

-Once done, select all object and paste as a new object, hide the background and save as a transparent gif on your computer.

Have fun dressing up. Here is my Holiday Costume. Happy Holiday Season Everyone!