This isn't really good practice, unless you revise your cookie on every page load to make sure it is still correct; e.g. restoring an ipad2 backup to an ipad3 will leave you with the wrong images for the next 10 years.
And an OLPC-style device might have different ratios if you're browsing using the e-ink display rather than the lcd.
And an OLPC-style device might have different ratios if you're browsing using the e-ink display rather than the lcd.