As a graphic artist myself who has a pretty huge portfolio I will tell you that a picture like your first one wont even count as a piece.
You need to do extensive work on an image in photoshop in order to get any college or customer to bite on it. You also need the original image to be high res. A portfolio piece should show many of your talents all at once and also show that you can use the program fully. I posted a desktop on here that i am still tweaking to us as a portfolio piece. I think now i have about 9 hours of work into it
Man, I really wish I had that picture.
The road/background/car/lighting/effects were all a pain in the ass to make them work together.
I got yelled at for a week straight to finish it, and as far as I’m concerned it still isn’t.
Still looks pretty cool though.