Wow! That's a really good game! It's nicely done, and, what is most important is that the solution is always deterministic. I'm really impressed!
One bug I have found is that once some cell is solved, you can still right-click on this value in other cells, and select it as a possibility for other cells in the same row. This should not be possible.
Also let me suggest a small change, which, I feel, would make the gameplay a little better:
I'd prefer the game to allow me to right-click wherever I want, even if my choice is wrong. Only left-clicks should count as mistakes. Another change that would have to go along with this, is that if there is only one (say) Lion remaining on the board (because I right-clicked all the others), it should not automatically become my final choice (I mean, it should not automatically get left-clicked, as it gets now). I should have a chance to select another Lion and unselect this one, and finally, when I'm really sure that the choice is right, I would left-click the Lion to commit it to the cell. Then, if I'm wrong, I would get a missclick info.
One reason for this suggestion is that it would be a solution to this:
When you play a game like minesweeper, which is also a "constraint-based" logic puzzle, you are able to temporarily try things by marking tiles, without actually committing to your choice.
With deductoid, every click you make is absolutely final -- you see immediately whether you are right or wrong and and basically you are told the correct answer if you got it wrong. That makes it not quite as fun as it might be otherwise i think. just my 2 cents.
Another reason is that if now you want to right-click some image but you have a muscle tick or something
and you accidentally click its neighbour, you might 1. get a mistake penalty, whereas a chance to correct your muscle tick would be more fair, 2. get an unfair piece of information, because if your right-click was a mistake, then surely... And then you either left-click this image and know that this was simply unfair, or try to ignore the fact that you know it is the correct image for this cell; both possibilities being a little spoilers.
Now it also happens sometimes that I just right-click some image, and there is a chain reaction of images getting automatically selected. I'd prefer to left-click them at my own pace, hiding unnecessary clues as they get used, and so on, and not suddenly waking up with a completely different board than it was a moment ago
I must learn for an exam at the university, but one must know what is important in life and what is not. The exam must wait now