Just seconding this request!
I too love Choices. But, when uploading a mix of image orientations in a portfolio item (most people do), it tends to make the page expansion / contraction a bit too 'energetic,' especially if one chooses to turn on the autorotation of the slideshow. Worse, the vertical images appear to be emphasized over the horizontal images (due to their larger magnification and level of detail) when this is not necessarily the case....
I notice that in the Choices demo site, most of the images used in the portfolio slideshows were all cropped square to avoid this issue!
Only on the portfolio display pages, couldn't the medium-size images be displayed to fit into a fixed-size CSS display zone (609 x 609 pixels) based on its largest dimension and then centered both vertically and horizontally pretty easily? (The pop-up lightbox slideshows would still remain full-resolution and adjust for orientation, though...)
This really becomes an issue for those of us who are designing websites for clients who are not particularly design-savvy. To expect a client who wants to be able to update his own website in the future to create two sets of images to upload (the second set having backgrounds to maintain uniform square dimensions regardless of orientation) and replace the medium images with an FTP client is just too complicated!
If the theme could display them fit to the square display area regardless of orientation, it would SO much easier for end users to add new projects by simply uploading all their images....