One thing that I found interesting about your artwork - it is very geometric in design. This gives it its own style that might not necessarily lend itself equally well to all subject matters - but makes for an interesting perspective.
I also notice and appreciate the shift between the warmer colours of the surface and the cooler colors of the bubbles and background. The one thing I'd suggest as a 'necessary' improvement is shadows. You have at least six light sources - presuming the bubbles/ gems are non luminescent, and these have difficulty travelling through objects.
The 'one' place that this would have been necessary is with regards the inner skull beyond the eye sockets. I'd have suggested that the far sides' walls be lighter - disappearing into darkness - as the current fill makes the skull look a little more 2D than it strictly needed to look.
I really like how you etched the signature or title into the drawing. It doesn't take anything away from it. The flowers could have been given a slightly different shade from the rest of the ground, like a shade of deeper red, or purple, with tiny contrasting pollen center.
Its an ambitious drawing though so don't mind me on the feedback. :c) Well done.
Thank you for the complements and wonderful critique! Constructive criticism is the best way to improve besides practice of course. do you know of any good techniques about the light sources I always have this problem :(
I am afraid not. :c)
My feedback is just based upon thinking things through.
that being said - before filling something in that has 3 dimensionality - consider where shadows (or semi-lighting) could be placed to enhance depth of field. In this case the inside of the skull could have "cheated" with a dim shading of the inner skill (perhaps light bounces upon the bone-white/yellow surface (scattering effect)).
As with all things - practice makes perfect - and it doesn't hurt to look at art shading to see how other artists do it. :c)