I don’t know if I’m stupid or something, but there doesn’t seem to be a way to add a picture to an existing Collection. That is a feature I think would be… useful.
This is an unfortunate side-effect of missing drag’n’drop support on Linux, which we are aiming to implement soon.
However, there is an alternative way using the local collection bar at the top of the center pane: You can either click the “+ Create” button, or just hit the “+” key, and then start to enter the name of an existing collection, which should then appear in an auto-complete list. Choosing an item from the auto-complete list will add the selected image(s) to the existing collection instead of creating a new one.
The same can also be achieved much quicker by using the numeric or manually assigned collection shortcuts (right click a collection to assign one), even if that doesn’t always scale to large numbers of collections.