From your description I would think that nothing went wrong at all. I mean, what did you expect, when you changed the SVG node's "SVG" property value to "No Image"?
Unfortunately, it is not possible to use the "SVG" property of a node to select a PNG image that should be displayed in that node. (While this is inconvenient, it actually makes sense - after all, the property is called "SVG".)
If you want to represent a node with a PNG image, follow the advice in How can I use my own node symbols in yEd?
If you have one or more palette sections with custom symbols, you can use the palette's "Apply" and "Apply Type" actions to change the visualization of a node. More precisely, select the node you want to change, go to palette section with the template you want to use, right-click the template in the palette to open its context menu, and choose "Apply" or "Apply Type".
