How do you make an icon for deviantart?

Photoshop

You will need a program such as Adobe Photoshop, Corel Paint Shop Pro, or GIMP to make icons. They must be exactly 50 x 50 px, either a .PNG, .JPEG or .GIF file, and must be no bigger than 15KB.

To use it on deviantART, find the 'More' button on your page, next to Submit Art and Edit Page. On the drop-down menu, select Edit Settings, and click Identity on the left hand side. There you can upload your icon!