I'd like to take half portion of an image, from top to half the height, then copy this part into the remaining bottom part and flip this by 180 in order to match the upper side. What is the an easy procedure to do that? Thanks!
Highlight the marquee tool and under the "Style" drop-down select "Fixed Size". Type in the width of your image and then half of the height of your image (note: if your default unit is set at inches, make sure that you type the "px" after your numbers in the boxes.). With snapping on, click the image and slide the marquee to the edge until it snaps in place.
Then copy that selection, paste it, drag it down to half way down the artboard. Then go to Edit → Transform → Flip → Vertically.