It's hit and miss. Sometimes the submit button appears, and sometimes it doesn't. I've tried both Firefox and IE, and the same problem exists in both. I complained to Bigstock administration, but they had never heard of it.
In one batch of three, two of them could be submitted without any problem, but the third refused, even though I did everything the same. The clue is that the green check mark fails to appear at the top, even though there is a green check mark beside all the individual items.
My interest in this is more a matter of IT curiosity rather than anything else. Sales are so poor at Bigstock that uploading would hardly be worth the bother, even if it worked well. The second time it happened, I just deleted them, and decided not to try again.