If you're unsure about what type of tiles to make, just look at some graphics of other platformers to get some ideas. If you know the type of environment you want to create, you can look up ref pics of the separate elements and graphically synthesize them.
What you have is a good start, some basic tiles, but almost every platformer has some kind of brick patterned tile, do something to make yours visually stand out so it doesn't fade into the realm of the generic. Colors play a role in this, and right now you're colors are good but could be amped up a bit for dramatic effect. If you create a strong contrasty balance between the background and the foreground, then you'll clarify some readability issues. I read the dark brick tiles as solid as well, but they seem randomly placed. Are you doing like a fade to black under the platform or are they just shadowy variation tiles? I like the little skull tiles, I think it adds character, and would love to see a 2x2 tile version of it.
And does the player sprite have to be 16x16? Ideally it would be bigger than the tiles themselves, but if it has to be constrained there are certain tricks you can do to make the player readable in a number of environments, mainly by using bright colors. (think mario or cave story).