Firefox resizes all images pictures and wallpapers to fit in firefox window by default .No one can view or detect the actual size of image in firefox without chaging the firefox setting. You can also change this setting in your firefox browser Here’s how
Type about:config in Firefox Url bar ( where we type www ) and Type Browser.enable in Filter bar and Hit Enter.
Double click “browser.enable_automatic_image_resizing” and change it to False and you are done, now you can view actual size of any image.