vertical image as k2 item image not taking up whole width

I have several k2 items to present with a vertical image. But I don't want it to take up the whole available width (625px), as they get too big. I'd like them smaller (width 450px).
A solution I found which doesn't satisfy me is to insert a "normal" image through the JCE Editor instead of using the k2 image upload function. It works: the image shows up correctly above the preview text and the "Read more" button. But it doesn't have the features the k2 item image has, such as: Link to the item, k2 image caption and the modal window pop-up when clicking on the image.
How can I insert a vertical image using the k2 item image upload form, adding the desired image width?

Hi

I do not understand at all Please show me the page You are talking about and tell me how do You want it.