Tough!
This was a pretty har one in places...
Some of the pictures I saw the dot right away, others I had to stare at the screen for a really long time. And the easy/hard ones were distributed fairly evenly over the entire game (as opposed to all the easy ones coming first).
Perhaps it's just a matter of personal perception and those pics I found dead easy would totally stump someone else?
One thing that bugged me was the total lack of any indication of progress. Somewhere along the middle of the game I started wondering how many more pictures were to come. And even after playing the game through, I couldn't say how many pics there actually were.
If you make a sequel, I suggest a pic counter (X/Y) and perhaps a timer.
Also, if you keep the general difficult as high as it is, a Skip funtion would be nice.