Here's a guide I wrote...
Guide to Getting Your Image Online
Have The Image On The Computer
First, have your completed, final image on your computer. Make sure you can find it later.
Find A Place to Upload It
To upload the picture you can do a few things:
1. Use a signature/image hosting service like sigup.com - Good.
2. Use ISP given webspace - Good, you have control over it.
3. Use webspace you own - Good, you have control over it.
4. Use free sites like Geocities, however, when you upload the picture, rename the extension to .txt - Bad, not as reliable, and you may be breaking terms of service on some free page sites.
5. Ask someone if they are willing to host the image - Ok, but the person could remove the picture later.
6. Upload it to a forum (get admin's permission to use on other forums too!) - Ok, remember the forum could close or something.
7. Beg Planet GameCube to host your image - VERY VERY BAD! They can't really afford the bandwidth, and asking is a waste of not only their time, but yours as well.
Link It!
Find the URL of the image that is uploaded. Copy it. Paste it where it needs to go for your signature or avatar. You should be good to go now!
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Hope this helps out!