transparent colors in windows applications in c#

[hosseinghazanfary]

hi dudes, i developed a windows application,i put a rectangle picture as a background pic for my form,in that pic out side the rectangle is collored by red.in my form i set that color as tranparency color,so it would show only that rectangle or any other shape as a form,it works well in my own computer,but whenever i try to use it in another computer which also .net framework is installed there,that color does not hide and the transparency does not work,it shows both the rectangle and the backgroung color,what should i do? i'm using C# in visual studio.that application is written in .net framework 2.0, thank u all for ur help.