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)