I haven't noticed any problem saving a PNG on images I have tested, but I did notice that when the WebP image is huge/complex, the file can be very large and take a long time to generate/save (JPG 6.7 MB vs. PNG 25.5 MB). If you notice any pattern to the problem, could you let me know at jscher2000@outlook.com or start a new issue at https://github.com/jscher2000/Save-webP-as-extension/issues/.