Des Plaines is pointless for photos, always have been and most likely always will be. However, there are ways to see images of items that they list, it's just not as simple as a single click.
For vendors besides Des Plaines themselves, images can usually be found on websites associated with the manufacturers. PSC has catalogs on it's site, Grandt Line can be found on the San Juan Models website, Tichy on the Tichy site, etc etc. Generally the manufactures part number is the same as the Des Plaines item number, with the addition of an abbreviated prefix representing the manufacturer's name.
For Des Plaines, most items in their O Scale America line are from tooling purchased from Custom Finishing years ago. I have posted pages from the Custom Finishing catalog on WT before, probably in the US Dabblings thread. I also believe that Yorkshire Dave posted that pdf again in the thread about resources for Red Caboose gp9's.
Found one of my posts, this is in the Red Caboose thread. The post includes the attached Custom Finishing parts list at the bottom.
Not all OSA parts are in the Custom Finishing catalog, as they originated elsewhere or were developed by Des Plaines themselves. But most of the OSA range is shown in the catalog pages.
Also, P&D carries OSA parts, which include the old Custom Finishing items. They will be listed on the P&D website under Custom Finishing though, not O Scale America. The P&D item number prefix is "CUF" rather than "OSA". Also, P&D now has photos of most items they stock, so often one can find images of parts there even if purchasing from Des Plaines. They both carry many of the same parts.
Hope that helps.
Jim