Once upon a time, you could restore a products missing icon by placing an image in the Manifest folder & renaming to match the Manifest product name and simply add a .jpg extension. 

That no longer works, and I noticed the items missing use .webp format instead and wondering if that's the problem.

How is it done now?

Maybe try *.png?

I aways use the png extension. JPG never shows up within Daz Studio panels unless there is also a png file with the same name. JPG is used for the Texture folder

Good to know.  Will give that a try.

Still no luck. File size of the png perhaps?

The dimensions of the thumbnails are 91 x 91 pixels, by default, although I am seeing 214 x 214 pixels in the Library.

Product images are usually stored in the support folder, not the manifest.

I'll check there then.

In any event, since this a long time issue, seems like an opportunity for a script or utility product for some programmer.

That worked. A slow process to be sure.

Textures in Daz aren't just jpg. You can use a variety of formats. When testing textures I've even used the psd file not that I would use psd for the final product. 

Good to know. But as with all things DAZ, getting it in the correct spot was the key. I was given mis-information years ago.