You can find some icons in this file directory (for the Windows 8 OS, anyway):
computer ------Windows (C:) ------ Program Files (x86) ------ Steam ------ Steam Apps ------ Common ------ Spiral Knights ------ rsrc ------ui------icon
Steam Users:
you can right-click "properties" in your games list on SK, then press the "browse local files" button.
Be sure to NOT MODIFY that file as it is in the game files. Copy/paste the image (or file) into whatever program you use to edit images, and then save that file into a personal folder (something in your MY DOCUMENTS file, or similar), and then upload to wiki from there.
Most icons on the wiki are 256x256, png format.
This method gets you specially shaped icons, such as the one for the Blooming Prize Box.