It's true that bandwidth is often the bottleneck on busy sites, but I think you have an error of one magnitude there. I'm terrible in dealing with these big numbers, but calculating with round and rough numbers: gigabit ~= billion bits per second ~= 125 million bytes per second ~= 122070 kilobytes per second. 100*100 / 122070 ~= 8,2%.

Editing myself: And that's when they download in the same instant. If the 100 users download the image randomly within just one second, the 8,2% drops to less than one tenth of that. Well, there is of course the disk seek times et cetera, but this bandwidth-wise this is what a gigabit can take.