Can't get internet photos to show in correct full size

[Xp4me]

I have a 24" 4K monitor (3840 x 2160). I'm using win 7 pro and in the Display Settings > (set text size (DPI) I have the choice of smaller, medium, and larger. I have to use the larger setting (150%) or else I can't read most of the text or make out photo thumbnails without keeping my face 15 inches away from the monitor. (No, I don't need glasses)

Everything is fine this way except when I click on an internet photo and then click a second time to view the image in full size. The image is always too large, not actual full size.

I've already tried changing the settings that I can think of. It seems that I'm pretty much screwed, it's either having to lean into the monitor to read text or having to download every photo and then open it in Windows Photo Viewer to view the actual, correct full size.

Is there a way to setup win 7 or Firefox browser to show the real, actual size of a photo when double clicking on the image?
Thanks guys,
Bruce