You can find a lot of ideas like that on pinterest. If you're not on there, a lot of examples come up when you google "DIY gallery wall" and look at the images.
I would definitely move that big painting away from the lamp (probably ASAP)--the light could probably damage the painting being that close (and the warmth from the light too), and it looks like the lamp is blocking half of it!
And here's a useful tip on how to hang a gallery on a wall
http://roost-home.blogspot.com/2010/09/another-take-on-gallery-wall.html

I appreciate the link. And you're absolutely right, the lamp is blocking the painting which is why I need to do some other arrangement. Thanks!

I would stack two paintings on top of each other. Take the one from your mantel and put it on top of the larger one. Keep 3 inches between them.

Switch your sitting components over. The chairs on the window wall and the sofa on the full wall. You'll find out easier to collage a gallery wall above the sofa. Unless TV viewing is the reason why the sofa is placed the way it is.