Even if the reader doesn't understand "auto" the text-align:center will get it centered properly. Using them both together as you have done works well.
I'm not sure about the height:800... If it is viewed on a screen smaller than 800 (landscape?) wouldn't that clip? You could add a max height/width to a generic img tag to prevent that:
img {max-height:100%; max-width:100%}
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