A DOS 3.3 slave diskette should be placed in the disk drive. (Refer to the DOS 3.3 manual for information on slave vs. master diskettes.) Assuming that the disk interface card is in slot 6, type .PRINTER 6 to LOGO; this command boots the diskette, trashes LOGO and the user's workspace, but doesn't damage the High-Res screen buffer.
Now that the Apple is running BASIC with DOS 3.3 in its memory, it is possible to save the High-Res screen buffer as a binary file:
BSAVE <filename>, A$2000, L$2000
To view the picture, type:
HGR
BLOAD <filename>