Photo editing

I have 26 photos of individual garments that are between 3-4MB and 4000 x 3000. I need to have the garments extracted from the photos and placed on a neutral background with dimensions of 650 x 475 and a file size of under 300KB.

I have added examples - the first 2 photos are the full size images, and they need to look like the 3rd picture. Some garments have more than one photo e.g. separate front and back photos that need to be combined as in the example, whereas others are individual. There are 26 photos in total.

I created the example but it is not something I usually do so it took me ages. I am looking to save some time.

EDIT - attached sample images. Summer dress 1 and summer dress 2 need to be placed side by side on a white background with total image dimensions 650 x 475 and a file size of under 300KB.