If the files will converted to png in the apk, its size will huge!
For example: I use 100 jpgs which are together 30mb, if they will png's in the apk then apk size will 200MB+.
If you images are photographic-like images, Jpeg is the format you'll have to set on export (via the image editor of C2), then C2 will not export the PNG, it's as simple as that.
However, If your images aren't photographic like pictures, then PNG is your best best for quality/size, C2 does use PNGcrush on them so they are smaller, while the same quality (loseless compression), however you can still set them to PNG8 so they have less colors in them, which result in a smaller size (lossy in that case, but smaller noneless).