you can also make a blog post on your character page through the lodesetone. and upload the image to your blog post. and get a URL to use like NintenPyjak64 suggested.

also here is a list of macro icons not viewable in the default selection window. like previously stated, just use the /micon "ICON NAME" command line. it won't show the icon in your User Macros window. but only when placed on the hotbar/XHB.

Available macro icons