Clone+Stamp+Tool

Clone Stamp Tool
Copy an image Go to photoshop File - New Paste Get clone stamp tool from side toolbar - ckick it - a cicle appears - chose a part of the picture you like. Click ALT key - cursor changes to a different cursor. Click left mouse key - this has created a sample. Now drag over where you want to change, Repeat as many times as necessary. If the circle is too big, just change the brush size in the options toolbar.

This can be used for skin imperfection, changing the cloudy sky etc.