A few things jump to mind:
1. the letter font is all capitals - the words suggest early readers who generally use lower case only. Also a school friendly font would also be useful.
2. the words and picture combinations would need to be quite clear. Generally you are restricted to concrete words only. i.e., easy to do "dog" not so easy to do "sunny" or "fast". In those case you need images that must be inferred.
3. It would be cool if you had more of a game aspect to it. Build up something, if you make a mistake it comes crashing down. Or some other game element.