Pixiv and Production I.G. hosted an illustration contest for original characters for Gargantia on the verdurous planet and multiple winners were announced on the official site. There are a total of 12 characters selected and they will make some kind of cameo appearance in the last episode.
by Konatsu (Aki)
by Ryuzaki Ichi
by Ahiru xx
The entries were collected on Pixiv from April 26 to May 12. To see all the entries, click here. 12 winning characters will be refined for cameo appearances in the last episode.
UPDATE: 0res found them all and posted an album. Thanks for the share!