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If you're using Neia's career tool, if you click on the title of the branch (Main, any sub branches) you'll get the opportunity to change the icon and in-game picture. The dimensions are also listed on said page.
Small Edit: As long as you have the dimensions, you can use anything as an icon. I, personally, have downloaded things that add more icons to my game. Something like
this is what you want, I imagine.