Out of the 4 of your piece
this one is the only one that isn't a logo and photo manipulation of a picture.
You put good use on that stink steam that is coming out of the bug but the bug itself lacks things.
I can already see from zooming in that you must of use the soft brush to draw the majority of the picture, I can tell you now this won't give your image any sense of sharpness on the left side of the image "which is the bug face" (which also stands out).
The right of the image where the smoke is coming and the slime at the bottom is alright, it gives the image some sense of depth.
The left side (where the bug face is) is where you need work on, which also let me guess some of the tool you used that isn't giving your image justice.
It lack the entirety of the bug feel, my advice is looking through google image to find a good bug to refer to for ideas on the texture of the bug.
I recommend practice on hard brushes instead of soft brushes first. A hard brush allow contrast to be sharp while a soft brush allow a lack of focus on an area you soft brush (which gives a blurry look that is often abused to a point on being unbearable).
You were doing good at putting up a light source, but you stray off the mark by not giving shadows below the bug and not giving a direction of the light source in the background.
My advice is knowing the basics of art;
then practice what you can do on paper being the same on how you draw on a computer;
and lastly is to observe (I could of said practice but it wouldn't help if you don't know what to practice "ON");
so go to youtube to observe speedpaints, go zoom in on arts to see the details (this can help out on knowing how things are done), and lastly lastly come over to the newgrounds art forum (the art regs can give critique or advice or answer questions you need help on).
Just remember to keep all your art in your own thread that you have to create and to use the image upload button for every drawings that you are showcasing :P