The Nextgen Gallery1 is an excellent plugin for WordPress. It allows to organize series of images to galleries and these can be inserted into posts as a whole are just into the text as single pictures.2
The Nextgen gallery is updated regularly. I’m using it on several blogs and it seems to be pretty stable. The plugin is easy and comfortable to use.
Unfortunately, rare but still sometimes I had the problem that images do not show up in a lightbox if you click on them. Instead, they are shown on a separate web page in the browser which is not a modern look and feel. A Google search showed that this problem happens also to other people. On the Nextgen web page there is a solution3 for that but the first few steps did not work for me and the final steps are too complicated. Thus, I looked for another solution and found a pretty easy way.
In your WordPress dashboard go to Gallery -> Other Options -> Reset Options and click on “Reset all options to default settings“. In a German version of WordPress this is Galerie -> Andere Optionen -> Standardeinstellungen wiederherstellen. Please note that you will loose all your special settings that you probably had set before.