Stick artist: in front of a new artist tag next time and it will. Also read howto:artist/howto:pixiv to figure out how to add new artist entries so they'll appear using the find artist button in the future.
New tags will always start as generic tags. You have to specify copy:, char:, or artist: when tagging the image to change the tagtype. For example, even with the artist entry for ookamimikoto added, you would tag the first image you uploaded by the artist as "artist:ookamimikoto" so that it would gain that tagtype. Then, for future uploads, the tagtype is set as artist.
Incidentally, for the artist entry, you don't want to use the profile image. Use the source path from the image upload, in this case: "http://img41.pixiv.net/img/ookamimakoto/"
