Currently KoreaPost offers “My own stamp” (나만의 우표) sheets with postage stamps only in the sheets, with the exception of one very large “seal” in these sheets. Three colourful examples of the possible configurations can be found on the ePost website. In these three sheets all the smaller stamps are actual postage stamps, but can show a few of your own images, while the large center or left side “stamp” is a large seal with no postal value whatsoever.
It used to be different. Here is an example from my own collection from approximately 2006. This example shows the opposite of today’s practice: all the postage stamps in the sheet are standardized stamps, the personalized part (in this case a photo of Mt. Halla on Jeju) contains only seals.
From a Korean blog comes this listing of some other examples from that era:
But that was then. Today there are several possible combinations. Here are two of them and more versions including no longer used (older) types of sheet combinations/examples can be found on the KoreaPost website:
First and second example of current My Own Stamp configuration options.
The second configuration could lead to a sheet such as this one as sold from a Park Chung Hee fan website:
While the first configuration would create this style of sheet as shown on Orangebada:
A page showing the whole thing step by step can be found on blog.me (in Korean only):
The way to create such sheets is to go to the ePost website and start creating your sheet. Here is a screenshot from the ePost website:
The price is (depending on the numbers and the specific configuration chosen) somewhere between 7.000 and 10.000 won per sheet. In practice it is next to impossible for foreigners to do this without having both a very good grasp of Korean and a local bank account to be able to pay for the result. Probably best to have a Korean friend or acquaintance doing it for you.