The difference between luscious moist lips and those that are dry is that moist lips reflect light. To give a dewy gloss to any set of lips using photo editing software, all you have to do is add a bit of shine. You don't need an expensive app like Photoshop to this. Any photo editor that lets you add layers to a photo should work well. If you don't already have such an app, GIMP, Paint.net and Krita are all free to download.
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The Basic Process
To add some gloss to a set of lips, open the photo in your graphics editor and select a color