This is actually most likely rather version un-specific. I'm in 626 though, trying to grab the Cset from an image I'd just ripped, and I find myself unable to select another picture to grab from. There is no visible File or Open button to choose another image from. Either that, or the actual ripping screen is influenced by the lack of color from the image, and none is visible. Even changing images to grab from in the Tiles Pages leaves us with the same image to rip from in the Palettes area with no method of changing images.
... or at least no obvious one. I'm going to try pressing "p" in the main ZQuest area.
EDIT: Nope. Same image; no way to change it to another image that I know of.
EDIT2: Ah, see, what's happening here, is that the entire File Button (Among the other buttons.) are being made invisible by the palette that's being ripped. Its color scheme is seeping into the GUI's, and making it invisible against the background.
The image is attached. See if you get a black screen with dark orange frames when ripping a Palette/Cset into the Main Palettes area.
(BTW, Props to you if you know where the image is from.)