sorry about this

You don't need to apologize for questions. There are probably many other girls planning to rush who also are confused. For the picture, it depends! If it's a headshot obviously your clothes won't matter as much. If it's a full-body pic, I'd advise that your entire outfit be on point. Wear something that shows your personality and personal style, it helps sororities get a better feel of who you are. If you are not a "pearls" girl, don't force it, but if you are, show it! I wouldn't do a sleep shirt picture. Try to find time to go out and have a silly photoshoot with a cute outfit and looks on point, and pick a super cute pic of you.
I wouldn't wear dresses every single day. The first days in particular. I would wear a cute top / shorts (not jean) or even a cute romper. Backless dresses are fine so long as they are not obscene. Remember that you are dressing to impress girls, not boys, so try to keep up with latest fashions. A little secret, accessories are super important! It helps us remember who you are, so put a lot of attention into your necklace / bracelet / earring choices (but don't overdo it, it has to match your outfit obviously). The "what to wear" guide really does help a lot with how to dress, if I was you I would closely try to match the "casualness" level of the outfits those girls are wearing.

^^^the sweat stains comment couldn't be more on point. CONSIDER THAT ALL WEEK lol.