Recently I got a question from one of our partners for a field in which a user can select multiple images and also have a overview of the selected images. Of course there are a few available fieldtypes but there was a requirement to select the images in one dialog window. A example of this was the TreelistEx field, but that field only shows the images as tiny icons with the name of the image next to it.
So I made a copy of the TreelistEx field and changed the xaml which shows the output in the content editor. Now you can select multiple images in the media selector dialog like this:
And the output shown in the content editor looks like this:
You can download the module on the Sitecore Marketplace