1. Pull down the File menu.
2. Pick "new."
3. Click the "+" sign next to "Advanced Options."
4. Select "Transparency" from the "Fill with:" menu.
5. Click "OK."
You now have a completely transparent image, to which you can add additional elements. In order to preserve the transparency for use on a website, you will need to save this image in GIF or PNG format. JPEG images do not support transparency.
Alpha Channel Versus 100% Transparency
Advanced programs like The Gimp and Photoshop support the idea of an "alpha channel," which allows every pixel (dot) to have any level of transparency, from completely opaque to completely transparent and anything in between. This is useful because it creates much smoother transitions. And the PNG file format supports alpha channel too.
The good news is that Internet Explorer 7 and Firefox both support PNG's alpha channel transparency very well. The bad news is that some older browsers might not (they will show the image, but they will not show the transparent areas correctly). The alternative is to use GIF images. Unfortunately, GIF supports only 100% transparency, so you will lose some elegance when using a transparent GIF instead of an alpha cnanel PNG.
For additional information, see how do I make graphics for my site?
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