PNG is a lossless format. It is compressed and heaps better than BMP, but it has its uses for small or exact assets like ui elements and not for photorealistic renders or actual photos. If you transform a png of a rendered master image into jpg you can go down orders of magnitude in file size. While you can go down like x3 or x4 from BMP to PNG, you can go down another x5 to x20 from PNG to JPG.
For jpg specifically, just try it out. Make a row of sample pictures of your content and since the compression in jpg is given as a number up to 100, just make a 100, a 95, a 90 ... and so on. Then mix in your original and see for yourself how small you can go, until you notice and how far you can go even further till you care. jpg is *very* good at compressing natural looking stuff or stuff that was made to look like it might be natural. Pixelart and comics/anime are unnatural in this context. (note, 100 is not lossless, but even this placebo setting can be half as big as a png)
WebP is available in lossless and lossy, so depending on the content of the image, the same rules applay. Pixel heroes -> lossless. Photorealistically rendered heroes -> lossy.
In other words, there do be reasons, why your digital camera stores those ZillionMegalPixel Pictures in jpg. That is the format you want for stuff that looks like real things. For stuff that looks like my little pony animation with big same color areas and sharp lines you might get away with lossless compression, as this compresses fantastical and you do not want to be fuzzy around the sharp edges, and for pixel graphics, you want to keep the pixels intact, so lossy is bad for those.
Just try out with different samples of your images what is efficient.