Hi VictorSeven,

WEBP can be used with Ren'Py (see docs) and is generally a good choice. Do take note however that it isn't always the best tool for the job. Given a state-of-the-art compressor like MOZJEPG, the size difference between JPEG and WEBP will be negligible. The far more interesting point will be the composition of your renders as both formats struggle with (as in compress less efficiently than their counterpart) certain details.
That being said, the best approach would be to use a tool like pio to pick the ideal quality setting for each image individually. Ideal in this case being the lowest possible that still retains the perceived quality of the original image (see structural similarity).

I let pio have a go at the 3.0 GiB of images contained in v.0.1.4 and the results were as follows:

• 336.1 MiB for JPEG
• 326.5 MiB for WEBP
• 305.4 MiB as best of both worlds (e.g. picking the smaller file of each of the above)

Switching from losless (PNG) to lossy compression can shave 90% off the size without sacrificing quality.

Oh, and while you're at it: you may want to encode the WAV files to OGG or OPUS to save another GiB of space.

Hope this helps!

Hordragg