Let’s start with the acronym: “Joint Photographic Experts Group”. This is an image format invented by an expert groups of photographers. It’s good for photographs, which have their own “fingerprint”: a vast array of (mostly) continuous blotches of nuanced shades. More over, these photographs need to be transmitted to human beings (at the other side of the screen), who are used to looking in the real world where these photographs are taken. This, by the way, also explains why they have no transparency: photographs do not have transparency.
So when you have a logo or a sign, which tend to be:
- Sharp divisive lines
- Few colors
- Shades which need precise reproduction
JPEG is horribly suited to the task. You will end up cranking the “lossiness” down (usually it’s called “quality” and you’ll crank it up), so it will take waaay more space than it should — and still end up looking like crap.
Here is a Star of David, in PNG form:
The size of the file is <5KB.
Here is the same, saved in JPEG with a quality towards the high-end:
What we got for a nearly 3-times increase in file size is horrible quality.