There is no "PDF format" for pictures. PDF uses JPG, JPG2000, TIF, and others within its format.
The PDF can be edited with Acrobat Standard or Professional under most circumstances (yes, there's a cost). However, the originator of the file can disallow any editing and require a password to make changes.
You missed the part of "... without a bunch of expensive software".
What shows up in a PDF may be a reduced resolution version of the actual image. PDF is designed not to allow isolation of the image without the (expensive) full version of Acrobat.
Using an inexpensive image program such as Compupic can allow cropping images from a screen capture, but only if size and resolution are not important.