There's a variety of programs and apps that can resize pictures. I like to use Irfanview on a PC:
https://www.irfanview.com/
The forum software seems to set the uploaded images to 800 pixels across by default.
iPhone pics sometimes display upside down because of how they record orientation data.
You can use a photo hosting service such as Photobucket or Flickr, but that may cause your pics to be gone later if terms of service change or you stop subscribing to the service.