[15.13] Can I pass pictures or other special data to my XObject?

Yep. As noted in the XDK, parameters declared of type P in the message headers are pictures. On the Mac these will be PicHandles (**** Windows? ****), and they can be used in QuickDraw calls as normal. On the Director side, picture parameters can be used in connection with the lingo picture of cast property:

-- if the XObject method mDraw is mDescribed thus: -- XP mDraw me, myPicHandle
-- you can pass it a PicHandle like this: mDraw( myDrawXObject, the picture of cast blah )
-- similarly Xobjects could return pictures: -- PS mReadTIFF me, fileName
-- which you could use thus:

  set the picture of cast blah = mReadTIFF(myTiff15Object, "myPic.tiff")

(before anyone asks, I'm not aware of any XObjects that allow for importing TIFF files!)