An easy way to insert pictures into FreeMind is by using Node / Set Image in the right-click popup menu (or Alt+K).
However, pictures in FreeMind are a preliminary feature:
If a node contains a link to an image, you can replace the contents of the node with the image by using the Set Image function. You can drag and drop several image files into FreeMind, select them as multiple nodes, and them turn them into images in one step by choosing Set Image.
A more technical and not-so-user-friendly way to put pictures in the nodes is by including the HTML. You have to start the node content with the tag <html>.
E.g.: