Because JPEG is a horrendous format for these types of pictures.
Using it is a mark of an inexperienced graphic designer.
Use JPEG if you photographed a real life object with a camera. ALL other images (for the web) should be PNG.
Oh, and the reason it's horrendous is that it makes the images blurry - and larger! And you can no longer realistically edit the image once JPEG touched it. (For example change a color: use a color picking tool, can't do it once jpeg touched it.)
I disagree. There are enough cases where a JPG image looks the same but has a smaller file size. There is no reason to not use JPEG in that case. You are overexaggerating with the "blurry and larger" part. Using PNG just because is a mark of an inexperienced graphic designer to me. Know your options and choose the right one.
Just keep the original work file (which surely is not a PNG either) and change that if you have to.
For a non photo the only time jpeg would be smaller is if it's a dithered image. And since you shouldn't use a dithered image anyway, I maintain what I said.
Using it is a mark of an inexperienced graphic designer.
Use JPEG if you photographed a real life object with a camera. ALL other images (for the web) should be PNG.
Oh, and the reason it's horrendous is that it makes the images blurry - and larger! And you can no longer realistically edit the image once JPEG touched it. (For example change a color: use a color picking tool, can't do it once jpeg touched it.)