You will need to Upload the photos to a Photo Sharing site like Photobucket when want to attach photos to threads in the forum.
So I am going to try my best to explain this using PhotoBucket. Now into/ about the Avatar as well.
The Avatar photo size will need to be reduced in size to 144 X 86 pixels or 198KB or which ever is smaller.
Which is a very small photo and a you may also need a photo program to reduce it.
Many free photo editers are available online. I am not sure what windows comes with never use them.
When you Click On User CP
it will open many options including then and look for Edit Avatar
click on that.
Then you will see how to insert your small photo for the Avatar.
The box looks similar to this:
You have 2 options there upload to Photo bucket or from your computer in this instance.
From the Photobucket site, once that photo is uploaded, to the right in boxes you can use the IMG
box click on that.
then (copy & paste that IMG link/info) back into the 'URL" or option1 in the Avatar box.
and hit save changes.
Or again in this instance you can upload the smaller pic from your computer.
Hope this helps.