Both Tony and Longedge are probably right. I do use on occasion some 4x3, some vertical orientation of photos as 9:16 and 3:4, some with differenr AR such as scanned postcards and crops but not too many of them on a 16:9 timeline and have no problem with the produced slideshow in the m2ts format. If mp4 is needed then a handbrake conversion could be the answer.
Years ago I created svcd’s using software before there was cyberlink. Found that the image size must be 704x480 ,not 640x480 and not 720x480 for the proper aspect ratio of 4:3 to be displayed. I accomplished this by Create a new image in say photoshop or gimp to be used as a frame. Crop the original image to whatever. Size to whatever to fill the frame. Flatten it. Export or save it as a new file. This means each image is of the same size for a slideshow. This same procedure may help studioksr if she has lot of different size images.
Maybe different sizes and different AR of images can cause problems but I haven't found any using the slideshow templates.