That's pretty small. 1800 x 1200 works out to only 2.16 megapixels.
If the site is automatically rescaling the pictures and you are not losing the margins I wouldn't worry about it and just let the site do its thing. If, however, the site is simply zooming in to a small portion in the middle of the picture I could understand that being unacceptable.
First option would be to go into the camera settings on the phone and see what resolutions are offered. It may not downscale that far though. If it does, resnap thepics. You might have to try some alternate camera apps to find one that offers alternate resolutions.
Another option is to put the photos on a PC and use software to resize them to meet the abysmally poor standard of 1800 x 1200.
Even better, find another real estate site which has a more modern standard.
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