Before digital imaging, PIs could feel reasonably confident about the data that were being presented. In this day of age, however, with the advent of scanners and Photoshop, images can be readily manipulated. This article http://chronicle.com/weekly/v54/i39/39a00102.htm mentions the new trend for journals to be on the lookout for pictures that have been manipulated. Note, there are distinctions between changing the contrast of a whole image and changing the contrast of only on lane in your image. As research becomes more competitive, we must be vigilant about who works for us and the data being presented (before it goes to be reviewed).