I know how to straighten an entire image. But what if there is a picture frame on a wall that's crooked? Can it be straightened? I'm using Photoshop CC the lastest version, on Mac Sierra.
One way: Make a selection of the picture, put it on its own layer, show transformation controls, make the picture slightly larger, then straighten the picture frame.
As Derek said. Or if you don't want the picture to be larger, select it - transform it to straighten it up then use the clone tool to fill any gaps left in the wall.
Or select the picture and transform - warp. There are tons of ways... if you post a screenshot we'll come up with even more .
If it's a picture frame on the wall, I would select an area larger than the frame and transform that - less retouching of where the original frame was.
Is the picture in it's own layer? Or can you select it and jump it to it's own layer (Ctrl/Cmd+ J)? If so, you can use the Straighten Layer option:
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The combination of Derek's and Dave's responses should easily do the trick.
Make a selection of the area
Use New Layer Via Copy (command J)
Straighten and enlarge slightly to cover the other image
You may need to make and/or soften the edges