upload it to some image provider or webspace, and link in that thread
one has to be 512x512 for future logo stickers, 32x32 for current icons ingame (SQ etc.)
It will be included into the game via patch later, from time to time they release a bulk of logos with it. ingame you cannot upload the pics, and you have to wait until the logo is patched into every client, then you can select your own corp logo for free in the corp setup.