Are you really referring to DICOMscope 3.0? ... because the current version is 3.6.0.
Here are my answers:
1) The difference is that the latter is a DICOM image (Secondary Capture) and can, therefore, be viewed with most DICOM viewers
2) I guess you are referring to the IHE MESA CPI test cases: The .pre files are GSPS (Grayscale Software Presentation State Storage) objects; the .res are DICOM images. See documentation.
3) A frame is a single frame in a multi-frame DICOM image. An image is a DICOM image SOP instance (i.e. a DICOM object/file that has an image SOP class and, therefore, contains pixel data).
4) This can be checked using the "Create dump" button in the toolbar of the browser. See user manual for details.