Renpy just uses images (or animations), so any program that can create artwork can be used. It highly depends what style you want. I assume your more looking advice on render programs. If that's the case, there are 3 options.
Daz: Main one used for visual novels on here, as it's easy to pick up, has a massive asset library, and is relatively speaking quick in creating artwork as you don't make your own assets. A fair amount of assets are pirated on here, but for very specific assets or theme, you got to pay up. Can become rather expensive quite fast. Most assets (especially characters) have some form morph modifiers to slightly change them, but you are still limited to assets sold on the store (or pirated on here). Rather bad for animations.
Blender/Maya/Z-brush: 3 different modelling/sculpting programs. The results can be rather similar, it's just what the artist is convertible with. Since blender is free, it's the most popular one used on here. Provides complete freedom in what you can make, but has a steep learning curve, and takes a lot of time to get results. If you want to create a very specific theme or setting, one of these programs are your go to.
Last option is honey select. Very limited in what results you can get, but very quick and easy to use. You can create animations from the get go as well.