Is it that what way it is on every screen shot? If it is it looks like the refresh rate it bad. I seen these on cheaper displays that half the display is fussy.
What version of OS X are you using? Older versions saved to PDF, which was clumsy to work with because of the automagical scaling stuff.
Tiger by default saves to PNG which is a lot easier to work with if you want the original bitmap, and on older OS X you can still copy to the clipboard (include ctrl in your keystroke combo) and save out using whatever graphics program you prefer.