Best way to convert JPG to USDZ.

Did you Google search with

convert JPG images to USDZ

You should find a lot of tools, some free, some paying, some with 21 days trial…

If they’re all the same dimensions then you could just take a UV mapped file (obj, fbx or glb containing just a plane), and use the conversion tool to convert that to a USDZ assigning the diffuse texture. a small script could create this for all of your images in no time at all. If, however, the images are all scaled differently then you’ll need to generate the meshes yourself by reading the image dimensions first, and then convert to USDZ.

The Roxy Art Market does exactly what you are talking about.