Maybe you’re confusing an image thumbnail with a high-res, zoomed-in photo.
A zoomed-in photo is actually just fine for screen viewing (which is the intended purpose, unlike, say, large prints).
For example in this recent topic I see photos at a resolution of 2560x1920, which is sufficient.
Downsizing takes place in the browser and is automatic, so you shouldn’t need to do that yourself before upload.