create smaller thumbnails
 

I want to create from every picture two thumbnail sizes. So that i can show on an external page the smaller thumbnails and in photopost the bigger thumbnails.

I have seen, that there are mini thumbnails, but i don't know how they work, and what they are for? Are they what i search for?

You can not create 2 thumbnails in Photopost. There is only one thumbnail and you can set the size for that in admin upload options as well as specify whether you want mini or regular thumbnails