Here's a quick and easy way to move an image to the top of a stack. When a stack is collapsed, clicking on the stack icon in the top left of the image will expand the stack. As you hover over each image in the stack an icon will appear in the upper left. For example, if you have three images in the stack you will see 2 of 3 and 3 of 3 for the second and third images.
If you click on that icon for any image (other than the first image in the stack) that image will immediately be moved to the top of that stack!