Merging two sprites (png files)

Merging two sprites (png files)

Hi, 

 

First of all I hope this is the correct pla e to ask my question, due to an college assignment I have to develop a game with libgdx the atari 2600 classic combat. 

 

I already found a good looking spritesheet but don't know which software to use  to edit the spritesheet i found a few like piskel but it didn't really help me out can anyone recommend one?