Here goes.... it is difficult to explain with words...

Right click on the avatar you found. A box will come up from
your computer on where to "save" it. You have to decide on this yourself. Make a name for it..that you can remember and where you put it. My Pictures is a good start.

Once it is on your computer, with a name/file location, you
can upload it here. We have size limits, and sometimes the software here will adjust for that. Sometimes not.
If it is "not" then there are adjustments you have to make.
Our size is 85x 85 pixels. Most commonly, .jpeg is the best format to use.

Go to CP in the upper left of the screen, to your profile and click modify avatar. The upload directions are there. If you fail in some way, go back and try again. Once you do one, you will see how the process works. You cannot break or ruin anything, so just keep trying.

Most of the functions here are step-wise and easy to follow.
There are avatar resizing free options on the net...if you Google that you'll find them if you cannot resize on your own computer.
Paint, is in accessories on MS PC's, and will resize your photos, etc..