You can request a skin by submitting it here.

Be sure to follow all the instructions & requirements!

Also some agario extensions like 'agariomods.com' allow you to submit your own skins too, you have a bigger chance for it to get added there. The problem with it is that only others with that extension installed (the agariomod) can see it.

You really can't do this unless you have a mod, like this one.

If you still want a skin use this:

Chrome, IE and Firefox

Download

Links

Chrome

Firefox

Apart from what @Actually Unnamed said, you can create a skin using the "skin creator" option (introduced in 2019). Here's how to create a skin in the mobile version of Agar.io:

1. Click the create button:

   

2. Write/draw whatever you want. You can change the background colour and the border colour as well:

   

3. Rename your cell and press save (here it shows 'free' because Agario gave me a free skin as a reward for something I don't remember now):

   

The method of creating skins is virtually the same as that on PC. (Also note that you can upload "images" through the skin creator but for that, you will have to install Agartool (an extension).)