I think there are apps that will do this for you, but the way I do it is to send the photo to myself. When I send it the email program asks me if I want it small, medium, or large. I always pick large, because it is still less than 1 mb, and it will work here.
Oh, and then I have to save the reduced photo that I receive back to my photo file. I can decide to delete either the large one or the reduced one after I post the reduced one to TradGang.