There are 3 image attachments posted in this thread of dimensions 1200 x 1200 px, 1000 x 1000 px and 800 x 800 px. You can use them determine how your monitor+browser combination displays the images.
NOTE: Your results may vary due to your particular monitor display setting and browser choice.
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Image: 1200 x 1200 px.
My Laptop: MacBook Pro Retina
My Laptop Display Setting: 1900 x 1200 px.
My Browser: Firefox - fully expanded, sidebars closed.
SMF Software 'Page View' Setting - Image resized to fit topic: Downscaled 1000 x 1000 px image must be scrolled to see top/bottom.
Right Click and *View Image - Image resized to fit screen: Entire downscaled image can be seen with no scrolling.
Click Again on Image: Image enlarges back to 1200 x 1200 and requires scrolling to see top/bottom.
*In Safari, Right Click and select Open Image in New Tab.