Photoshop CS2 is free as Adobe released it for free a few months back. It’s very useful to this day. I say learn it, but GIMP is also a good choice. Try both out, see which you like more if you can’t decide.
But yes, an idea for posing is to get a reference picture up, it helps loads. Say you are posing a rifle wielding man aiming down the sights, googling “rifle aiming stance” will usually provide a helpful picture. You can always use yourself for reference as well if need be.
I myself use the Ragdoll mover ever since it came out, but physgun if you have much more patience to master it is also very usable. I think most the masterpieces posted here were done by Physgun so it is possible, just need to practice lots. Might be worth giving it a try along with the standpose tool, sadly for unknown reasons some models will just not work with it.
But these are not that bad, they show a good grasp of basics, and your willing to listen to this advice shows promise, too. Keep it up, and I bet you’ll be making some fantastic shots someday.