Here is a link to info about recommendations at USC:
http://www.uscpanhellenic.com/letters-of-recommendation
As you'll see, now is actually not too early in regards to sororities accepting recs, but the deadline is August 15. You are off to a great start if you already have five! Someone familiar with USC recruitment is the best one to answer this, but for what it's worth, I send tons of recs to SEC schools as well as some in the ACC and few in the Big 10 -- it's typical to send them in the summer, after PNMs have their final GPAs, whether it's high school OR spring semester if they are already in college, either as a current student or one transferring in. Waiting until summer also allows any end-of-year activities/honors/awards to be included on the resume. (Beginning this year, though, I am making some modifications based on some great thoughts from some great ladies here at GC-- see end of post for explanation, after I've addressed your questions.)
In my part of the country, we advise two good recs per chapter. Yes, you do need to include a photo. A nice head shot or a classy casual shot -- think the style of senior pictures. No Glamour shots, trout pouts or swim suits! You want to present yourself nicely and you want to be recognizable. Sororities don't request a photo to rate you a 1 to 10 ; it's to help them remember you among the big sea of faces they meet during the week.
Finally, you need to provide your rec writers with a resume. Include everything, but emphasize leadership and service/volunteer work whether it is school, community, church or work sponsored. Though college GPA is what counts/is reported in the case of non-freshmen, most of the non-freshman PNMs I do recs for include a blend of college and high school activities on their resumes -- this just gives the sororities an idea of your interests, talents and background.
Hope this sheds a little light for you, but again, someone with direct experience with recs at USC may advise you better.
(Regarding the modifications: though I will still be sending recs in the summer, I plan to contact my chapter early about some PNMs and give them a heads-up that a rec will be coming. 1.) If the PNM is a
current student; letting my chapter know that she is already on campus may get her on the radar a little earlier, and 2.) if the PNM is not currently on campus but planning to attend a voluntary pre-recruitment activity, like a Panhellenic Preview/info session in the spring. (Some schools host these). Again, letting them know she will physically be on campus during the event may get her on the radar sooner.)