Dear Christina,
Thank you for the opportunity to see your painting, I remember seeing it in the Composition thread here
http://forum.portraitartist.com/show...threadid=2044. You have indeed taken on ambitious project for your first oil painting.
Although your questions relate to the issue of background "props", there are several more fundamental issues that I think you should look at before choosing the prop, and then in conjunction with it if you decide to include something.
Drawing. In looking at your reference photo, I have an impression of a slender, physically fit officer. In copying Mr. Paules's figure, you have given your subject the same physique as Mr. Greene, who appears to a larger and heavier man than your subject. You have also squared off your subject's shoulders, which gives a very different, and somewhat awkward feel to your subject's pose. In Mr. Greene's portrait, the accurate drawing makes the subject really rest on the table, and his shoulders follow an oblique angle, consistent with his posture. When two resource photos are combined in anatomically improbable or impossible fashion, you get this type of result. Your two resources are close, but not close enough. The only way you can correct this is to find a stand-in model similar to your subject in physique and size, light him the way the subject is lighted, and to redraw in accordance. You'll need to get hands, too, since your subject is looking for the glasses in Mr. Paules's subject's right hand.
In looking at the close-up of the head, I can see you are beginning to build form; your resource photo has a direction to the light, and I think you may want to spend more time modeling the values of the forms of the face, before doing lots of detail work; I think it works better to place features on a form that looks three-dimensional, rather than to try to add volume later on . It's also important to recheck your drawing for accuracy at every step of the painting. The hat is a strong element in the portrait, but it doesn't sit on his head the way it should - it's too tall, placed too high over the eyebrow, and too wide on our right.
Recheck the placement of the eyes, and the width of the head relative to the width of the face.
The horizontals of the brows and ears are considerably more angled than the horizontal of the nose and lips. I find it also very useful to consider the silhouette; it is a guide to where the planes of the face break, turning away from the front of the face to meet the ear; fortunately your resource photo gives you some good information here. Assuming you wish to keep the flag, check the drawing of the flag, as the stars don't seem to follow the folds of the stripes. Try turning your painting and your reference upside-down to check; a hand mirror will help as well.
Value. Try to make the most of the shadow and light pattern provided in your photo. Using a three-value compositional sketch can help you make decisions about how to mass values - it's best to do this before you start, but it can help you if you get into a conundrum partway through, too. This way you can work out how the values should be distributed over the picture's entire surface. I see that you have toned down the white stars and stripes in the flag considerably, but they are still a very strong eye-grabber. You can minimize this by softening and losing some edges in the flag, especially along the table.
Color. It looks to me like you are going for a blue-orange complementary color scheme (?) and that your oranges are quite desaturated compared to the blues; so I think you have a good direction going with respect to balance. You need to decide whether your light source is warm or cool so you can utilize different temperature in your light and shadow, which will help reinforce the value pattern of your picture.
The halo effect you are working on is a convention that many painters seem to use, but it reads here as a light source, one that has nothing to do with the light on your subject. The crisp edge and white cuff are strong, too, but if you subordinate them to the subject's face, they can be an effective foil.
Lastly, I had a chance to briefly look at the reference photo link. As you are most probably aware, you may face copyright issues here, and should do all you can to educate yourself about them and whether they apply to you here.
Kindest regards,