In "the trade," we refer to it as "link rot." Sites die, move, reorganize, and so forth. While it's possible to set up a page that redirects you to the new spot (if any), it's not always feasible (for example, if the domain expires and someone else buys the name) and the owners don't always think of it.
Free software can be especially susceptible to this sort of thing. The designer loses interest in the project, and everything vanishes into the sunset.
The Internet Archive's Wayback Machine is often useful, here. And as Jon suggests, just hit up your favorite search engine to see if there's anything comparable or just moved. Occasionally, you'll also turn up a "mirror," which might be out of date, but still have what you need.
In this case, it's also important to remember that the files are just ZIP and RAR archives of JPEG files. While CDisplay and similar programs are certainly convenient, they're by no means necessary to the process.