Now let's start with a heraldry from Robert's article, as example. Open your picture in Gimp. Then, select the "Magic Wand" tool. Be sure to check the second "mode" in the bottom part ("add to selection").

Then, click with it in the area of your background. You will notice that it gets a dashed border. If your background consists of more parts (when you have a more complex heraldry), then mark them as well. If the border of your heraldry is also selected, then undo the last action (in the menu: "Edit > Undo") and lower the threshold in the bottom slider of the tool window.

Now it's time to add the transparency to the selection you made. Right click on the picture, and select "Layer > Mask > Add Layer Mask..." in the appearing menu.

In the following dialog, check "Selection" and "Invert Mask":

Et voila, that's it! Now just save the image, and confirm all questions ("Export", "OK", and you should be done)!

Preferably, you should use the "GIF"-Format, which preserves transparency!