Pedigree Forge includes a basic image editor, designed to help with some of the tasks you might want to do with source document images, like cropping. This can be accessed either from the Edit Image icon button on the sources tab toolbar, or from the Media grid on the sources panel:
The “Image Editing Dialog” will then appear, showing the current image.
If you edit an image (by cropping it), you are likely to have to change and “paths” drawn over the image, as they are placed based on the images size.