Hi Ioke, I don't have a ready-to-go answer, but... there is a plugin that inserts a single image from RSGallery2 in a content item. This plugin has just been updated by Radek to work in Joomla 1.5.x:
http://www.rsgallery2.nl/rsgallery2_modules_and_plugins/plgrsg2singledisplay_j15_highslide_and_formating_paramater_16662.0.htmlThat might be a start, the plugin doesn't seem to work when used in a Kunena forum message directly (pasting the part {rsg2_singledisplay:6,thumb,true,left} in a forum message) where it does work in a regular Joomla article. You could find out wether or not it is possible to use such a content plugin in your Kunena forum?
Another approach could be:
In Kunena posts it is possible to insert an image with BBCode, by using a weblink to the image e.g.
www.mysite.com/images/rsgallery/original/test_600x600.jpg. So you might use that and figure out how to create an extra Kunena button that does what you want it to do. Then you figure out how to supply the user with info when that button is clicked.
The image link to any RSGallery2 image needs the main location of the website, e.g.
http://www.mysite.com, the path to the images images/rsgallery/ and the type of image, e.g. original, or display, or thumb, and the filename. Combined it gives the complete path. You will have to show your user a list of available images, or perhaps galleries and their images, and based on the users selection get the correct filename from the database. If you are able to create a list of images, then you are also able to get the correct filename... and you can create the path.
I'd love to hear if you get it to work, or any part of it.