I have been searching for a "cheap" X-window snapshot application - xsnap (source-link) - seems to be the one.
Its a 16K bin and has this modest deps: X11, Xext,Xpm,Xau and Xdmcp.
The original uses xmessage to show help but this version is modified to use the overseen alternative to Xdialog: getgui pointed out by technosaurus here.
Attached the compiled bin as well as the modified source and a build script. If you prefer the xmessage version just download the org. source and use the included build script to compile it.
Usage: xsnap [-options...]
Where options are:
[-display host:display] Display to connect to.
[-bd borderwidth] Color of border
[-bw borderwidth] Width of border
[-geometry geom] Geometry of snapshot window
[-icongeometry geom] Location of icon.
[-region geom] Region of screen to be copied.
[-iconic] Start up in iconic state
[-name name] Passed to window manager for name of window.
[-nograb] Don't grab server during specification of region.
[-xpm] Write to output in 'xpm' format.
[-xwd] Write to output in 'xwd' format.
[-file filename] Write output to file 'filename'.
[-convert app] Apply conversion program 'app' to image.
[-stdout] Write output to stdout.
[-noshow] Don't display in window.
[-show] Do display in window.
Button bindings at runtime:
Button 1: select region of capture or framed window
Button 2: click on window to be captured
Button 3: cancel snapshot
Key bindings at runtime:
p/P save snapshot as .xpm file.
w/W save snapshot as .xwd file.
z/Z enable snapshot compressing through gzip.
n/N do not compress nor use any other conversion.
other show help on key bindings.
There seems to be versions out there that also save to png and jpg but I could not find the source .
Well - found it. Second attachment is the version 1.5 (32K). Saves to jpg, png, pnm, ppm, xpm and xwd. Just changed the font to fixed in source-file to make him run. And in this version no need for xmessages or getgut - the help/action is build in - just right mouse click on captured image...
Update 070712: Source is gone so posting it here for reference