1. Get IrfanView.
2. Make the thing you want transparent a unique color, one that you didn't use for the picture, and save it. eg. If you have a Pikachu, with yellow, brown, black, and red, don't use any of those colors. Use something like green.
3. Take your saved file and open it using IrfanView.
4. Select Save As...
5. The filetype must be PNG. Open the PNG options.
6. Check "Transparency" or something of the sort for the PNG. Don't check the ICO by mistake. <_<
7. Save. When there is a pop-up having your image in it, and it says "Select transparency color", click the unique color that you picked. In the example above, it is green.