Having just gone through the pain of creating an avatar here are some additional notes and explanation of the gotchas that will hopefully help anyone else wanting to create their own avatar. You will need to read the post on 'How to post a photograph FAQ' in conjunction with this.
This technique uses programs that come as standard with Microsoft Windows.
1. Open the original photo or image that you want to use for your avatar.
2. Use the Microsoft Snip tool to take a screen shot of part or all of the image. When saved this will dramatically reduces the size of the saved file.
Snip tool before snipping:

Snip tool after snipping
3. Save the new image from snip tool as a .jpg file
4. Open the image with Microsoft paint
5. Resize the image to no more than 100x100 pixels. Maintain the aspect ratio if one if the dimensions is less than this, 100x43 in this case.
Using Paint to resize the image:
6. Save the resized image as a .jpg and upload to Photo Bucket as per the instructions in the FAQ
7. In the Photo Bucket library you will see that your image has a number of different names and locations.
8. Choose Direct and copy the file location to notepad, strip off the ~original that Photo bucket appended to the file name
Photo bucket library:
9. Copy the photobucket file location without ~original as your avatar url see the FAQ for this. Don't try and rename the image in photo bucket.
10. All being well you should now have an avatar!