Hi Frederik,
You can import images by dragging them into your wiki or through the import button (sidebar -> tools -> import)
You can also keep images external and reference them using their path on the hard disk
For both cases, the images will show up in the editors "image" dropdown - look for the image icon in the editor toolbar. Then simply click the image you want to include in the tiddler
A quick How-To if you want to leave images external:
create a tiddler, give it a descriptive title (it will be the tiddler containing the image) ... add a field _canonical_uri and insert the path to the image in its value field
depending on the image type, choose the correct type for the tiddler, then save the tiddler...