2. There are a lot of editors for Joomla! and it depends on your needs. JCE is considered one of the best.
3. You need to upload the image to the gallery folder. You can use K2 media manager and just drop the image to the item gallery folder. K2 items galleries are saved under /media/k2/galleries.
the issue is that I don´t know how to add further pictures to an existing gallery.
At the moment I have to delete the whole article and create a new one with a new gallery .zip file when I want to expand the gallery with more pictures.
And I´m a little bit disappointed of the K2 media manager because I don´t see anything it can do more than the standard joomla manager. I can´t manage the K2 article media with it....
OK, to make it clear, the suggested way when you want to edit a K2 gallery is to delete it and upload the new one. Another way is to configure K2 Media manager to open at the directory K2 saves the galleries. If you do so you will be able to add more images to an existing gallery. This assumes that you know the id of the item in order to find the correct folder. To configure media manager open at media folder go to Content->Media Manager->Options and set the Path to files folder to "media". Keep in mind that even if you resize the image SIG Pro will resize it again regarding it's settings. Finally note that this can only be done in the backend not the front-end.