The first step is to actually have the picture on your computer as a file. It's not enough that you can see the image - that image lives in lots of places. You need to give the image a home on your own computer
(or "organized sand", to use the OTTish). To do this, go to the post with the image in it. The one with the hat. (Note - this works with any image displayed on your computer). Once you are actually looking at the image you want to invite into your organized sand (computer), you do the following:
--> real computery stuff! (works with most organized sands) --> step one
(Let me know if and where you get lost, and I'll walk you through.)
--> right-click (or right-mash) on the image. You should see a menu open up with some options.
--> left-click (or left-mash) on the option you want. In this case, it's the one that's closest to saying "save file" or "save image to your computer" or something like that. On my particular organized sand (OS), the correct selection is "save image as...".
--> You should then be presented with a box asking you where to give it a home. The file name should be already filled in. This is a little tricky to explain, so practice first without actually saving. You can do this with any image. If you double click on a folder, it will open (taking you "down" and letting you put the file INSIDE that folder). You can double click on folders that are already open... this takes up "up" and lets you put the file inside that
folder. The simplest thing is to put it in "Desktop" or "Downloads". But wherever you put it, you'll do the same kind of thing to find it later.
--> Once you've selected (double clicked) on where you want it, click the [save] button, near the bottom of the box.
If you did this right, the file should be on your computer. Next, you'll send (a copy of) that file to xkcd and tell it it's your new avatar. To do that, you'll need to navigate to your account profile area. So, log into xkcd.com (if you aren't logged in already), and go to "User Control Panel". The link should be up at the top left somewhere.
--> real computery stuff (but should be more familiar)---> step two
(you can do it!)
--> Click that link, and in the resulting page, click the PROFILE tab. Tabs are along the top (usually).
--> Click the "edit avatar" thingy. (Thingys are on the left... usually).
--> You should see your existing avatar. Under it, you should see a [browse] button. There should be a label that says "upload from your machine".
==> (xkcd is not enlightened. They say "machine" when we all know it's really an "organized sand".
--> Click that button, and you should see a menu box that will let you navigate to your new icon's present home. It works similarly to the one you used to give it a home in the first place. Once you find the file, click it and then click the [open] button.
--> ("open" is really not a good word for it, but that's what we're stuck with.)
--> When you click "open", the box will go away, and you'll be back at the avatar display. There should be a button that either says "submit to forty lashes" or just "submit". If you see "submit to forty lashes", back slowly away from the computer. If you see "submit", click it.
One of several things will happen.
Either the world will end, with raptors crossing the streams and rectalinear assumptions being made left and right, or Donald Trump will be elected president (pretty much the same thing), or your avatar will have been uploaded.
In the latter case, Contratulations! You have done <computery stuff> and have a new avatar!
In any of the other cases, simply follow the second commandment. (or is it the third?)