Did you try changing (replace myTImage with name of your TImage)
myTImage.Transparent:=true;
myTImage.TransparentColor:=clPurple;

If that is what you mean, change background color?... Gif, jpg, png and most anything else than BMP are formats that you cannot use canvas with. They are controlled through other objects such as TJPEGImage in jpeg.pas. GIF was only introduced after Delphi 2006 and don't know which class and unit it comes through.

http://delphi.about.com/od/graphics/a/delphigif.htm