Copyright and Trademark are done outside of this site, at the appropriate US Government agency with jurisdiction over these two actions. Look there, google it, I did.
I had a ebook cover designed. I did find out, after I started, that I was obligated to pay more money for the images the designer used to make my design (stock photos, off the web) This added 50.00 to my design cost, which was then 250.00 overall, not bad as I'd budgeted 300.00 for a good, professionally designed cover.
You work with your designers (many at first then 6 finalists). You'll find a range of abilities and experience. Many will be slow or even unresponsive, a few out of their depth, and a few will have skills that can produce a great cover. Find them, work closely with them, chose them as your finalists, then push them all to improve their designs. If you've chosen wisely (yeah, Indy) the designers will listen, alter their design and ask for more feedback. Its give and take, not give and you passively receive. The better you are at looking, saying what you like and don't like, the better image you'll get.

I cannot address the format issue. As far as trademark the designers are supposed to use their own original art. Let the designers know in the brief whether you will or will not pay for stock Art.

Only after you selected a winner do you have a chance to see the design filess. But, the viewing is limited. You only get the full design after you transfer the money to the designer. This was the first time that I saw that the designer, Alex_82, had uploaded files that were not editable nor could I use them for a book cover, my intended purpose. After a week of emailing Alex_82, the 99designs people sided with the designer and it was up to me to work things out. I ended up with no usable files for my book cover. As for the trademark, 99designs and the designer are supposed to give you the legal rights to the trademark and I think some designers try, but if there is a question of trademark rights you will be found at fault and not 99designs or the designer.

There is only one way to protect yourself with this company. Don't give them your credit card.