Best format and resolution is subjective. If your focus is on good quality renders then I would recommend the webp format in 1080p (1920x1080 standard HD). Webp natively works in Ren'Py, supports transparency, and doesn't lose nearly as much detail compared to jpeg. The file size is also smaller than png which will matter if you plan to have a lot of renders. For example, my VN currently has over 560 images and is just over a 1 GB. Only a handful of images in there are jpeg and png, the rest are webp.
However you don't have to output directly from Daz into the final format for the Ren'Py image folder. What I do is set Daz to output images in tif format which is retains all the details I need. I touch up the tif renders in Photoshop and then I use
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to convert all the renders into webp. To get a rough idea of the final appearance, in my signature are links to threads for my VN's, one has images in 1080p and the other in 4k (both are jpeg as F95 won't display webp).