I found this post about stamps through Technorati tagged Slovenia.
They were issued on the 23rd of September 2005 in a series about Gothic Art. The stamps, printed se-tenant, picture a detail of the Dance of Death fresco from the church of the Holy Trinity located in Hrastovlje. This fresco, realised by Janez of Kastav was completed in 1490. It is 90cm high and 630cm long. It features eleven figures representing various steps in life. They are all holding hands with a skeleton as pictured on the stamps.
What is quite impressive on these stamps is something that can not be seen on the scan. The stamps are embossed on the skeleton : when you rub your finger on them you can fill (feel) the bones.