Please put more work into this
There is a plot, but I think you should flesh it out more.
The graphics are very pathetic. The masks, the frozen limbs...the list goes on. Work harder on this and you'll have a much better response from other users. Try multi-layering, and work on mouth movements.
The characters weren't portrayed well. Since each turtle had his own developed attitude you should have played off 'em. You slightly touched Leo's character, but you left the others bare. This doesn't mean you should change the storyline, just add details that compliment each character. People appreciate things like that and reviews and scores will be higher.
General details--like shadows, leaky pipes, sewer rats, 'clangs' when weapons meet, 'thuds' when opponents fall, etc. would really help this flash.
Please also animate the characters, and put them in correct proportion. They were supposed to be in excellent physical shape, ja? Where's the muscle?
I liked April. Feet movement would have been nice, then legs, arms, etc. One at a time. Please don't move the whole body. It will look better and score higher if you don't keep characters 'locked' into one position.
Hope this review helps your future flash.