- Open your image.
- Select the area you want to make transparent.
- Select the appropriate selection tool from the Tool window or the Tools - Selection Tools menu on the Layer window. I usually use the magic wand/fuzzy select (Select contiguous region) tool or the Select regions by color tool.
- Click in the region you want selected. Use shift-click to add more regions/colors.
Tip: It's easier if you zoom in (View - Zoom menu) to see finer details of exactly what you're selecting.
- In the Layer window (the one showing your image), select Layer - Transparency - Add Alpha Channel. If this is blanked out then it's already done. This makes sure your image can store transparency data.
- Select Edit - Clear. This makes the selection transparent.
- Save the file.
Note: If you save it as a PNG file, be sure to select the 'Save colour values from transparent pixels' option in the Save as PNG dialog box.
And that's it.
NOTE: IE does not support showing PNG transparencies (alpha channels) natively. There are a few work arounds for this (theres loads to be found on google) - here's MS's solution. However, personally, I find the simplest approach is to save the image as a GIF (which all major browers support transparencies for).
When saving your image as a GIF from within Gimp, it will prompt you to export it - you can select either Indexed or Greyscale - however, on the next screen (Save as GIF) be sure to check the 'Interlace' option.