If you're using (or have) photoshop, perhaps give the free webp plugin a spin: https://github.com/webmproject/WebPShop
It has a very good compression ratio (up to 50:1) without losing quality. if the engine you're using doesn't support the webp extension, simply rename it to JPG/PNG (and etc.) after conversion.
Already using the WebP extension, but the thing is that Itch.io only allows up to 1000 files when uploading it for browser playability, but we have 1400+ in this version (next version will have a lot more files), so based on the FAQ and other things, the only way to fix it for us now is to wait for support to increase the file limit.
