Select an Title image for grid view with TV

I have created a portfolio. On the project page all images will be shown as a slideshow, starting with the first image. On an overview page an other image will be shown. This title image should be selectable by the editor.

My first idea was to use a custom field. But the editor can activate more than one image as the title image. It’s not user friendly.

My second idea is to use a dropdown select as TV. This can simply a list of numbers, i.e. 1 tp 5. But I need the amount of images in the resource.

How to get the numbers of images with an SQL query? Or is there a better way to do this?

What about simply designating the first image as the image for on the overview page? You can easily filter on that with the mgGetImages snippet plus I think that should be easy for an editor to understand.

But my customer don’t want the first image as the title. He want the possibility to reorder the images for the slideshow and select an other image than the first as the title. This is sometimes the second image, sometimes the third, sometime …