Scootermm - This is the most important advice you will get regarding this -
GET SOMEONE WHO HAS AN MFA TO HELP YOU WITH YOUR STATEMENT!
I am doing the same thing, and this is VERY important to do. Your audience is ONLY the profs looking at the work. PERIOD. They don't like it, you ain't getting in. So get some help. The person helping me spent 3 months working on a statement for her MFA thesis with Joyce Numanis(sp?). So take it for what it is worth.
Also, have them look at your work. There is little guidance out there on what to do really. Have someone who has done it look at your stuff.
I do assume you have a single 15-20ish image series you are submitting and not just 15-20 different images. Because if you don't have a series, and I mean a coherent single body of work all relating to ONE MESSAGE, save your money, work on it for a year and then try next round.