This is a bit OT, but I had a great picture of our cat, but in the background there was a monster Dust Bunny under the table that was just amazing.
So I just Photoshopped it out, and one of our kids was watching, and she said, "This is the way my Mom does housework!".
I do have the before and after pictures.
So the answer to this, is yes, you can do most anything to a double chin or shadows or things that you need to correct. Ice cream all over my grandson's chin, yes its gone.
Just remember to zoom way in and go slowly and carefully on a duplicated layer. Dupe the whole picture just to be safe.
Blur, clone, healing brush, liquify gently, go for it.
Hi, Dian -- I'm a Smudge Tool fan myself for smoothing out excess chins or other wrinkles. (But then many of the artists who do the weekly Challenges know that that's one of my favorite tools.)
Because the Smudge Tool can create a "soft" edge where contrasting colors meet (where chin meets the background, for instance), I usually "select" the subject first and do a copy/paste to a new layer, then clean up the edges of the pasted object with the Eraser Tool, before using the Smudge Tool. If the edge gets too soft, I can use a hard-edged Eraser Tool to tighten the edge back up. I start with a very low setting on the Smudge Tool and work my way up to what feels right. The Smudge Tool gives you the feel of pushing the pixels around. The Dodge Tool set at a very low level can sometimes be used to lighten the chin shadows prior to using the Smudge Tool. You would need to experiment with the Range on the Option Bar (Midtone or Shadow) to get the right result.
Depending upon the background near the person's waist and the type of clothing, you can also sometimes narrow a waistline by making a selection of both the background and the waist area, doing a copy/paste of the selection, and then using the Move Tool to gently reposition the selection in a bit (repeat on both sides). With the Move Tool selected, in addition to your mouse, which is sometimes hard to control for very small movements, you can use your arrow keys to move the selection in tiny increments, left, right, up, or down. The selected background covers up the fatter area of the waist :) When you're happy with the waist size, you can use a variety of tools to blend the selection with the original (Clone, Smudge, Eraser) so it's difficult to tell that the waistline has been altered. I've used this technique for a too wide jaw with good results, also.
And as Jane said above, ALWAYS work on a copy of your original. Create the copy BEFORE you start making any changes.
If you can get access to any one of Scott Kelby's "Photoshop Elements Book for Digital Photographers" there are two excellent sections (Retouching Portraits and Body Sculpting) that have made me very popular with the senior ladies in our circle of friends. :)