I would expect xdg-open command to use the same application that opens when I double-click the file in the default file manager, but this is not always true.
For example my DE is XFCE, my file manager is Thunar and my default picture viewer is Ristretto. However, xdg-open example.png
opens the example PNG file in Pinta. Why?
Best Answer
specific to your question, you could try this to set the default application associated with the png file:
you need find out what exactly the desktop file name of Ristretto. afterwards, you could check it with this: