2. update

 

The sprites need a lot more contrast and it's hard to tell what the ground is. Give the sprites blacks instead of the brown you have now; it makes them look dirty (as in covered with dirt) and it just sticks out. the ground could be helped with some texture or maybe a total change of material. You should add lighter highlights to your palettes and give the characters more dynamic poses. I don't know about you, but if I was being shot at by an automatic something, I'd get the hell out of there, not stand tall and straight as I'm riddled with bullet holes.

 

Also, about that automatic gun, generally stuff rotates, true, but the bullets usually only come out in one place. Unify the bullet streams, I'm trying to say.

 

The dithering on the health bar needs work (use fewer hues and colors maybe?) and the text isn't pixelled. Make your own; the text tool is for lazy people. The palette for the health bar border and the text is boring and unsaturated and will probably be lost in a background; I suggest a white or black, or something bordered with one of those colors.

 

Good luck improving these, and don't quit!

 

Leel and Bifur (wb biffy!) gave you the right info about dynamic poses. You can add some more interest and realism by giving the joints bends and trying to make the poses yourself, IRL. If it feels weird or you fall over, you'll have to change the character's pose. The IRL test helps determine balance and whether the position is feasible. The enemy with the big gun on the right looks like he has some serious balance issues; see how he's supporting all his weight on one leg and how it's not bent at all? That pose would be uncomfortable and near-impossible holding a gun that way. This stuff isn't really hard, but you do need to think it through and maybe make a fool of yourself standing in different poses for no apparent reason.