personally i only consider two cases: iris and nostrgram. iris explicitly recommends a 1500x500px image but will happily accept any size and will present the image **centered in the middle** so as long as you use a pic with the main subjects in the middle you'll end with an image "compatible" with several clients. just be considerate of the size in bytes please, i've seen full hd sized high res gif animations in excess of 50mb and at this point it's just ridiculous