So it turns out the one that said "Oekaki Kensaku" was for an Oekaki Kensaku contest, and is the doing of a Sou Takasato if this is anything to go by. Unfortunately, getting the image in its original quality seems to be impossible, short of time travel. Oh well. At least I found a textless version.
The only other mystery about this is who the heck thought it would be a good idea to have "Oekaki Kensaku", meaning "drawing search", be a character's name!