Listing sites reject oversized images, crop them unpredictably, or take an age to upload them. A few minutes of preparation avoids all three.
A listing where every photo is the same size and shape looks considered. One where images jump between portrait, landscape and square looks thrown together, and buyers read that as a signal about the seller.
The first photo does most of the work. It is the one shown in search results, and it is usually the only one a scrolling buyer sees. It deserves more attention than the other nine combined.
Photographs straight from a phone are often four or five megabytes each. Ten of those is a slow upload on a poor connection and provides no benefit, since the site will shrink them anyway.
Around 1600 pixels on the long edge suits nearly every marketplace and uploads quickly.
You can, though many marketplaces discourage or forbid it. Check the rules of the site you are selling on.
No.