It is possible to import images to be used as nodes.
Aside from that, it is also possible to display images as part of a node's/edge's label. To do that, select the node/edge/label in question, go to the properties view in yEd's lower left corner, find property "Icon Properties", and click into its value field (typically displaying the value "No Image"). This opens a dialog with further options. In this dialog, click the button next to "Icon" and choose "More" from the displayed popup menu. This will open a file chooser which can be used to select an image file from your hard drive.
See also "Help" -> "Example Graphs" -> "Visual Features" -> "node_labels_with_icons.graphmlz".
Additionally, it is possible to convert existing nodes into labels of other nodes to create complex symbols.
