There's a few issues that hurt this game, but all of them are fixable. If all of them were addressed, I'd rate this at a 4. Otherwise, good job.
First, as others pointed out, the slow movement of the frog is just brutal. It really slows down the game unnecessarily and makes some of the levels that require a lot of backtracking to be painful.
Second, reading the dark grey on black for the levels and the congratulations at the end is a poor color selection. Please choose a brighter color.
Third, the game is just too short and too easy. I found level 12 to interesting, and the curve up to that point was fine, but then the game just ended. I would have liked several more levels at or above the difficulty of 12.