Hi Laura, We are definitely hearing you and agree that easy drop shadows would be a fantastic enhancement to Ribbet.
There is a way to create a drop shadow for a transparent logo right now. You need to first create a Blank Canvas with transparency ticked, then drag on the transparent png or gif from the Photo Basket. Next click the Cog icon (up the top right next to Undo/Redo) and select Combine All. Lastly apply the Drop Shadow frame with transparency ticked.
The ability to tick on a drop shadow on any element would be a very valuable feature though, and we will look at including this in the future.