The "Enlarge Image" bar will show up on any image that is posted with a width higher than 700px. This is done for the cosmetic benefits of using the site - the tables will all be the same width the whole way down the page, unless the user chooses to enlarge a photo. However, even if they're shrunk down by the board, it is still loading the full-size image; so a super-high-res image will still hog up bandwidth, even if it looks a reasonable size.
Some of us therefore use thumbnails or previews linked to an external full-size, so people have the option to load it if they want. SkylineObsession wrote a handy Photobucket walkthrough on dealing with this (though I'm not sure if it's still up-to-date, as I don't use Photobucket).
Er, so, long-winded answer: keep your images under 700px to be safe
