Have you ever been on an internet forum and noticed someone posting a rectangular image at the bottom with their name on it? Have you ever wanted to be the envy of your fellow forum members? Have you ever wanted your posts to not look so boring? Do not look any further.
Steps

Step 1. Look for the rules that the forum may have about signature images
They generally do not allow images with a width of more than 500 pixels, or those with a file size of around 1 MB.

Step 2. Choose the image you want for your signature
It could be a funny picture you found, an animation from a TV show, a funny clip from a webcomic (be sure to follow the attribution rules to avoid copyright infringement), or a YouTube video.

Step 3. Upload the image
You can upload the image to your own server or to a photo sharing site like Photobucket or Tinypic.

Step 4. Copy the appropriate URL to the image

Step 5. Go to the 'Account Settings' section of the forum and click on "Edit Signature" or something similar

Step 6. Insert the link in the text box

Step 7. Now enjoy your new signature that you designed yourself
Advice
The forum may have a section dedicated to good forum signatures. If you make one fun enough, you might get to see it there
Warnings
- Again, check the rules before creating your own signature. You don't want to get in trouble with administrators.
- Be careful not to put things in your signature space that could be offensive.
- Some forums do not allow you to put your signature on posts if you are a new member. And some forums don't allow signatures at all.