Lopsided images can be straightened out by rotating them to the right or left by the appropriate angle.
The first step is to measure the degree of lopsidedness using the Measure tool. You'll want to take several measurements of the horizontal and vertical lines that you are most interested in making level. This might be, for example, the horizon or a side of a building.
After obtaining the image's degree of lopsidedness, choose the command Rotate > Rotate Arbitrary and enter in the angle needed to straighten out the picture.