check_http reports the size downloaded, so I’d guess the time is for downloading the object of that size. Looking at the one I have running here, it reports 26kbytes which is just the size of the html without the images. You could create yourself a page with a large image to test yourself if check_http shows it to be any different to the equivalent page without the image.
I’d guess you could try doing something with wget, but I wouldn’t recommend put too heavy a plugin on your main Nagios server unless it’s run only very infrequently. I’d run it somewhere else and push the results back to your Nagios system using nsca.
My (production) Nagios server is groaning a bit at the moment thanks to some hefty scripts I use as plugins, so I’m looking to do the same here, and I’ll move the MySQL database off the Nagios server as soon as someone frees up the hardware to run it on