As your first well I would say this turned out pretty decent and the mood is quite nice ^^
But there definitely is a need for lots of improvements :
1.The cropping (cutting process) of scans is not good...lots of jaggy lines and lots of parts where the tip of hair is missing or those white parts around the edges of skirt,hair,feet should be removed !
the texture at the bottom was certainly a good touch but if would have preferd it more if th color were a mix of dark blue and glowing light blue...oh this is just personal preference of course XD

3.Always have the overall quality of your work control and dont let it drop too much...Even if you do a masterpiece but the quality is lower than the standard your work wont be appreciated.
using high quality brushes (refrain for oversizing the brush since the quality would decrease dramaticly ) and vectoring the characters or using special plugins in order the render a scan would help alot !

4.Always provide a link of the materials you have used for your work specially scans and stocks to avoid misunderstanding and also to let everyone have a brief idea of what you have done !


Hope to c more works from you since you seem to have good imagination ^^