Duvik's advice is very good & he's right: this is a great first shot!

I'm not a GIMP user, so I can't be specific ina ctions, but I think the best and first improvement is dropping the neon, bright color scheme. (Especially in a pulpy adventure, which generally has a moody undertone.) Either recolor or play with the color saturation & hue (not sure how in GIMP, but in Photoshop it's under the Image>Adjustments pulldown menu).

I think you'll be really happy with even this one change.

Oh, and be sure to post updates as you work so we can see & help with your progress.

Take care!