Scanned images often have a slight rotation because the scanner can not so easily be controlled in the placement of the sheets, and there is not enough time to devote it our entire life. In other cases, the scanner’s tray process several pages at the same time and sends them doing its best effort which doesn’t mean that results are ok.
As a result, we have pictures as the graph shown on the right, twisted.
The most practical solution is to use Picasa, Google’s free application for image management.
Image is selected from the corrections side panel, then choose the “straighten out” option.
Then “apply” and go.
Additionally Picasa has its own virtues, although this and the option to export a large amount of documents to an specific size and format are the only ones I have used.
Here you can download Picasa.