1: Find a nice gun graphic, or something that passes for part of a gun. 2: Open photoshop (or Krita, Gimp, or Irfanview). 3: Add gun to the sprite where the player is holding their fist out. 4: Save. Or do it programmatically, with the player object and the gun object, and you can have the player "hold" different guns when they find them scattered throughout the world.
Part of being The Dev is working with what you have, and building something great out of it. Even if you're not an artist.