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What's wrong with jpeg?
It's okay for photos, because photos include a natural blur, but when you deal with pixel-precision, jpeg is a nightmare, since it always adds a certain blur. Try making a simple BMP file, red and green, with a sharp contrast between them. 2 colors, no more. Then save it as jpeg, the border between the colors will now be blurry, consisting of many color gradients.
I generally stick to PNG when it comes to all images, digital art in particular, but generally with all images. PNG saves each pixel exactly as they are. BMP does too, but BMP is ancient and huge files.
Thanks for the advice.
Lost transparency? That's why I always keep the editable file.