This one sounds straightforward: Add a PNG resource file, for use with sprites/tiles, from inside the Solarus editor.
Place a button 'add' in the sprites editors, select a file from anywhere on the local volume. Then, Solarus copies it into the required directory, and assigns it a name matching the resource ID a user provides.
That would be simpler than manually going to the resource directory, copying, renaming it, and ensuring that it matches.
Also, a 'commit' button, for editing the tile/sprite page, would be nice, so that changes occur only after pressing a button; and a verbose response ('change committed') would be good too. I would make both of these latter two, options enabled by default, that a user could disable in prefs.
Ok for the PNG file import feature.
I am not sure I understand the commit button. There is already a save button, what is the difference?
A separate commit function, when adding tile selections to the tile selection pane, in the PNG viewer. i.e When you select an area, and make it a tile, you can click commit to store it, and get a verbose dialogue. Because there's no internal response to making the object, it's hard to tell starting out, if what you're doing is working.
An internal PNG loader would, of course, be of higher value.