I read this Blog from Albert Gareev, thought that's great but there's another way where you can use not only marker packs but also any pictures which is useful as described in my Mindmaps for managing test projects blog.
For example, I wrote the names of my team members in Word, Arial, 14pt, bold and took a picture of the name and now use them as markers. That's very useful when assigning projects to people so I can see at a glance who is working on which projects and where they may be stretched a bit thin.
In XMind this can be done by
navigating to your installation folder, for example
Drop your *.png or even *.jpg files in there.
Open "markerSheet.xml" in the text editor of your choice and add XML entries using the existing entries as a guideline. It's pretty straight forward.
Save and re-open XMind if you had it open before.
Now right click on an object and insert a Marker - voila, your new pictures as markers are now available. You are no longer restricted to pre-designed packs but can cook your own.