As per the topic,
how do I change the icons of the expand and minimise tree node icons?
See UltraTree.ExpansionIndicatorImageCollapsed, UltraTree.ExpansionIndicatorImageExpanded, et. al. (2008.1 and later).
There's no property in the tree to allow you to specify custom images for the expansion indicators.
You can use the tree's DisplayStyle to switch to a Windows Vista-style tree.
If you want to specify your own custom images, you would have to use AppStylist. You would go to the TreeNodeExpansionIndicator UIRole and you can set an Image on the Expanded and Normal (collapsed) states.