diff --git a/src/core.py b/src/core.py
--- a/src/core.py
+++ b/src/core.py
@@ -20,12 +20,12 @@ class Core:
 		self._ownMessages=MsgQueue(self._handleEvent)
 		self._guiMessages=MsgQueue()
 
-		imgPath=os.path.join(os.path.dirname(__file__), "..","images","7.jpg")
+		imgPath=os.path.join(os.path.dirname(__file__), "..","images","5.jpg")
 		self._frame=PIL.Image.open(imgPath)
 
 		self._guiProc=multiprocessing.Process(name="gui", target=gui, args=(self._guiMessages,self._ownMessages))
 		self._guiProc.start()
-		self._guiMessages.send("setCurrentFrame",(self._frame,))
+		self._guiMessages.send("setCurrentFrame",(self._frame.copy(),))
 
 	def setCorners(self,corners):
 		self.detector.setGridCorners(corners)
@@ -44,7 +44,7 @@ class Core:
 
 	def joinGui(self):
 		self._guiProc.join()
-		self._ownMessages.send("kill")
+		self._ownMessages.send("!kill")
 
 	def _handleEvent(self,e):
 		actions={"setCorners":self.setCorners, "setTresholds":self.setTresholds}
diff --git a/src/gui/mainwindow.py b/src/gui/mainwindow.py
--- a/src/gui/mainwindow.py
+++ b/src/gui/mainwindow.py
@@ -33,15 +33,16 @@ class MainWindow(tk.Frame):
 
 		w=int(self.imgView['width'])
 		h=int(self.imgView['height'])
-		self.img=ImageTk.PhotoImage(frame.resize((w,h),resample=PIL.Image.BILINEAR))
-
-		wo,ho=self.currentFrame.size # o for original
+		wo,ho=frame.size # o for original
 		widthRatio=wo/w
 		heightRatio=ho/h
 		self._imgSizeCoef=max(widthRatio,heightRatio)
 		# shift compensates possible horizontal or vertical empty margins from unmatching aspect ratios
 		self._imgShift=EPoint(wo-w*self._imgSizeCoef,ho-h*self._imgSizeCoef)/2
 
+		frame.thumbnail((w,h)) # resize
+		self.img=ImageTk.PhotoImage(frame)
+
 	def _createWidgets(self):
 		# a captured frame with overlay graphics
 		self.imgView=tk.Canvas(self)
@@ -69,6 +70,8 @@ class MainWindow(tk.Frame):
 	## Stores a grid corner located at x,y coordinates.
 	def addCorner(self,x,y):
 		self.corners.add(x,y)
+		log.debug("click on %d,%d",x,y)
+		log.debug("sizeCoef: %f, shift: %d,%d",self._imgSizeCoef,self._imgShift.x,self._imgShift.y)
 		if self.corners.canonizeOrder():
 			# transform corners from show coordinates to real coordinates
 			log.debug(self.corners.corners)
@@ -81,7 +84,7 @@ class MainWindow(tk.Frame):
 	## Redraws the current image and its overlay.
 	def redrawImgView(self):
 		if self.currentFrame and self.img:
-			self.imgView.create_image(2,2,anchor="nw",image=self.img)
+			self.imgView.create_image(240,180,anchor="center",image=self.img)
 
 		for corner in self.corners.corners:
 			self.markPoint(corner.x,corner.y)
diff --git a/src/util.py b/src/util.py
--- a/src/util.py
+++ b/src/util.py
@@ -22,7 +22,7 @@ class MsgQueue:
 			if self._queue.empty():
 				self._event.clear()
 			log.info(msg)
-			if msg[0]=="kill": break
+			if msg[0]=="!kill": break
 			self._handleEvent(msg)
 
 	def setHandler(self,handler):