A list of misc stuff to think about when using Gimp:

To get a logo from a site in Firefox:
Go to "Tools" and click on "Page Info".
Click on the "Media" tab.
In the list of pictures, look at each one until you found the logo (this one is Youtube).
Right click the location, and click "copy".

In the URL bar in Firefox, right click and click "paste".

Right click the image, and click "save".
Open it in GIMP.

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Make a GIF (no alpha transparency) into a PNG:
Open the image. Press (Ctrl+A). Then press (Ctrl+Shift+V).
Save it as a PNG file.

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Crop a logo to just the logo (ex., Youtube's logo):
With the select tool, select the logo.

Under "Image", click "Crop to selection".

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To increase working size:
First increase the image size (under "Image") [:

Then, go to "Layer", and click on "Layer to image size".

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Make a logo transparent:
In progress! Here is what I have so far:

Of course you start off with your alpha-enabled logo.
Using any kind of select tool, select the areas that are reflections, any non-logo part (that should still be there), etc.

Next, go to "Layers>Transparency>Color to Alpha".
Select the most common color (usually white).
Click OK.

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Change the background on an existing, transparent logo:
My suggestion is to keep them transparent. But if you are sure you want a bg color:
To change the background on an existing, transparent logo simply click on the background (The white one: ).
Select the color of your choice.
Then, go to "Layers>Transparency>Remove Alpha".