I spent some time trying to google it, but to no avail, so I thought I'd ask here:

I need to customize the "Add new item" dialog of K2 so that it meets the following requirements:

1) the choice of options is limited, compared to what is normally offered, e.g. the user does not see the Gallery tab

2) users can add images (multiple) anywhere within the text of the article

I'd welcome any suggestions how to do it either by oneself or with a 3rd party component.

There are some options in the parameters of k2 to disable some things in the front-end display. Gallery is one of them, I believe.

And users can add images anywhere in the article by default.

Othewise you need to edit the itemform.php if you want to edit it more.