Straightening something inside a photo

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

warmly/j

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:

1. Select the Ruler tool
2. Draw a line segment along the top of the object
3. Select Straighten layer in the Options bar

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.

Or select the picture and transform - warp. There are tons of ways... if you post a screenshot we'll come up with even more .

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.