I found JPG does not support transparency, the alpha value is always 255. I am wondering only png supports transparency?
png-32 supports different levels of transparency. Each pixel can have an opacity between 0 and 255, with 0 as completely transparent.
png-24 supports setting one color as fully transparent. Everything else will be opaque.
gif uses a color palette. You can specify that one color in the palette is fully transparent.
png-8 also uses a color palette but each color in the palette has its own opacity value, so it supports varying levels of transparency too.
jpg does not support transparency.