Submissions have already been accepted for this month. I believe they stopped taking them around the 15th, now we have the opportunity to vote until the end of the month. Next month it starts fresh and everyone can enter again!
When I first started posting pictures I had trouble also. You have to make sure the size fits the size listed for your picture type such as .jpg, .gif, etc. Unless your picture is that size it won't be accepted by the program here.