You just remembered? First thing I would do is establish the size of your canvas before going any further; dimensions of the game. How else will you competently create the best composition for this project? Rough in all the elements and evaluate how they sit with eachother; throw in a quick title logo, title screen options, etc - make sure you got enough room. Surely this game isn't set to such an extreme "landscape" orientation, being so wide. If unsure, make it 4:3 ration, that's more likely.
You might think about adopting a palette closer to the ref pic, yer a little saturated at the moment. I'm sure that'll fight the title logo for attention.