TOBEDONE
Quick information to help until we finish writing a page for this:
After opening a viewer, you can also link it to the Lister it came from via the "Link to Lister" button on the viewer toolbar.
(If using old toolbars, grab the button from Default Toolbars, similar to above.)
Linking is one-way by default. For two-way, so changing image in the viewer changes the active file in the Lister as well, turn on:
Settings > Preferences / Viewer / Standalone Viewer / Link both ways (file display selection tracks Lister-linked viewer)
If you'd like to always have viewers open linked (e.g when double-clicking a photo), see this forum thread:
Make Standalone Viewer automatically link when it opens
(We plan to add a Preferences option for this in the future, to make things easier.)