The image viewer can only be used to view pictures; it can't edit them. For that, you must use a picture editing application, like the GIMP image editor. To open a picture for editing in a different application:
Click Open With, or right-click on the image.
Select the application you want to use to edit the picture.
When you are finished editing, save it and close the other application.
The image viewer will detect that the picture has been changed and will reload it.
To delete a picture that you don't want any more, right-click the picture and select Move to Trash.
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