This can be used to your advantage by looking at your access logs for not only IP addresses that hit many of your pages, but that did so in a short amount of time. Basically, spambots will have a high number of hits, and a short time between hits. Since they cannot view them, there is no point in downbloading them as it only wastes the spambots time. Most users nowadays browse with a graphical browser, as well as browse with images turned off. If you have only a few small images, your users are much more likely to leave the images turned on.
Most users tend to follow a few common paths.
Spambots can be detected because they do not follow links based on the content of the link, like a real user would, but follows merely based on the placement . Detecting a spambot becomes easy by looking for that path. See the page on luring spambots in for some ideas on how to limit URLs to only spambots.