I recommend the Korean online photo printing service Photomon.

https://www.photomon.com/Member/login.asp

Similar to Shutterfly in the US, Photomon offers photo prints, photobooks, calendars, cards, etc. We had a 30x40cm family photo printed and delivered to the US Embassy mailroom. (I couldn't figure out how to deliver it to base housing.) Prices were very reasonable and service was very fast. Note that the website is only in Korean, but I was able to use Google Translate to navigate.