How to convert the background to transparent? [closed]

I would recommend this (just found via search):

1. http://lunapic.com/editor/?action=load
2. Browse for image to upload OR enter URL of the file (below the image)
   http://i.stack.imgur.com/2gQWg.png
3. Edit menu/Transparent (last one)
4. Click on the red area
5. Behold :) below is your image, it's just white triangle with transparency...
   [dragging the image around in your browser for visibility,
   the gray background and the border is not part of the image]
   
6. File menu/Save Image
   GIF/PNG/ICO image file formats support transparency, JPG doesn't!

If you want a command-line solution, you can use the ImageMagick convert utility:

convert input.png -transparent red output.png

Paint.net is a free photo-editing tool that allows for transparent backgrounds. There is a simple example on YouTube http://www.youtube.com/watch?v=cdFpS-AvNCE. If you are still on Windows XP SP2 and that's an issue, I would first recommend doing the free service pack upgrade. But if that is not an option there are older versions of Paint.net that you could download and try.

Quick solution without downloading anything is to use online editors that has "Magic Wand Tool".