Hi Tom. It seems like this would be a basic feature to add to Meta Slider. Between Tricia asking this question here, and "the ultimate solution" described in your own documentation ("you need to upload correctly sized images to begin with. The solution for portrait images is to add padding to the left and right hand size of your image as demonstrated below.") -- have you considered adding this function into Meta Slider? You're already doing so many cool image things, that adding padding of a specific color automatically seems like an easy answer here.
I realize I can use an image editor to manually add the padding, but this is quite a bit less convenient than just selecting the pictures I want, in whatever dimensions they happen to be, and for the slider plugin to be able to pad it.