Here’s a (very) quick hack to grab an image, resize it to span all your Mac’s displays and chop it up (messily) using ImageMagick - it splits things up into quite a few more images than you might need, but the point here is that this works for me with 3 side-by-side displays, not all the same width.
#!/usr/bin/python import AppKit,os,sys from Quartz.CoreGraphics import * span = 0 height = 0 crops = [] bounds = [CGDisplayBounds(screen.deviceDescription().objectForKey_('NSScreenNumber')) for screen in AppKit.NSScreen.screens()] bounds.sort(key = lambda s: s.origin.x) for (w,h) in [(s.size.width, s.size.height) for s in bounds]: span = span + w height = max(height,h) crops.append((w,h)) os.popen("convert -resize %d %s resized.png" % (span, sys.argv[1])) offset = 0 for (w,h) in crops: print w, h, offset os.popen("convert -crop %dx%d+%d+0 resized.png %d.png" % (w, h, offset, offset)) offset = offset + w