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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.