Selecting borders is such an individual choice thing. I spend more time doing that than anyone should!! You will get many suggestions, and you should "try them all out." Lay your quilt top down and lay the borders next to it. You will probably find that some really appeal to you more than the others. I usually lay mine on a bed, figure out the ones I think I want, and then walk away for 30 minutes or so and come see if I still like what I have selected. If not, I start over.
I tend to prefer two borders. Usually my outer border is darker than the inner border....but not always.
I really like your quilt! I am trying to decide what I would do. I am liking the idea of the first border being the green background. It would give the pictures the look of "floating." I am NOT good at selecting fabric, as I can't seem to visualize well, so....well, I am going to just not suggest a second color, though I would put a second border on. I will be coming back to this thread to see what others suggest. I just wanted to offer my method of selection....trying different borders out.