A few things:
When uploading, only need to upload at original resolution. Its a bit difficult to tell, since your sprite/effect/bg seem to be diff. size pixels. So whichever is smallest, upload so that 1px = 1px
Downsizing as done in the preview, causes NPA effects when the computer automatical AAs everything. Maybe just pick part of your image that you like and upload that for the thumbnail
Lastly, Im not sure how you are doing your lighting effects but they are adding a ton of colors so I'm guessing its not by hand, which won't pass for pj gallery.
I would suggest that you can upload a version of this scene w/o the lighting effects, in correct resolution with new thumbnail. Then add a link to your forum topic about this project and/or an external image of what it looks like now, so people can see how you intend to use it!
thanks for the detailed explanation, no the light is not by hand, sorry for the trouble.