I have moved your question from Feature Requests to Help as this seems to be the more appropriate category.
It is not possible in yEd to create new node or edge types. You can, however, import images (JPG, PNG, GIF) or vector graphics (SVG) to be used as nodes. To do that, first create a new user defined palette section ("Edit" -> "Manage Palette", then click "New Section") and then import new symbols into your section (in the palette manager select the section and click "Import Symbols" or right-click on the new section in the palette component and choose "Import Symbols" from the context menu). See also 'Palette' Tool Window documentation in the yEd manual.