Before and after only works when both pictures are in the same frame. Sent separately, nobody lines them up mentally.
Side by side suits a comparison, because the eye can flick between them. Stacked suits a sequence that someone will scroll through, and works better on a phone where width is scarce.
Images of different sizes are scaled to match on the joining edge, so nothing is stretched and nothing is enlarged past its real detail.
A small gap in a contrasting colour makes it obvious these are two pictures rather than one odd one.
The two pictures are scaled to the same height, so the edges line up. Pictures that are close in shape and size join best. If one is much smaller, the other is shrunk to match it, so crop both to the same shape first with Crop & rotate.
The picture you add first goes on the left, so add them in the order you want them to read. A small space of around 12 pixels separates them neatly. Save as JPG for sharing, or PNG if exact detail matters.
Yes, add as many as you like. For a grid rather than a line, use the collage tool.
They are scaled to match on the joining edge, never stretched.
No.
The first one you add. The second goes beside it, to the right.