Photo upload/download specs

[kennywhy]

I was going to post a listing for selling my Yaris. When I uploaded the photos for the posting the pictures were too small, and only went in a row along the bottom of the message. They were jpegs directly downloaded from my camera and into the website.
Is there any special setting on a camera to get a larger photo? What are the specs for having a large, full page photo appear on a posting like most of them are? Probably something simple I'm missing. Help please.
Thanks,
Ken