Guide
Four pieces on how the formats actually differ: what to use for a photo, what to use for a screenshot, and why a shot from an iPhone will not open on someone else's computer.
What JPG is
One format under two names, how it works inside, and why every re-save makes the picture a little worse.
JPG or PNG
A photo as PNG weighs ten times more; a screenshot as JPG looks grubby. Why that happens and what follows from it.
JPG or WebP
WebP is lighter and does more; JPG opens absolutely everywhere. The choice comes down to who is going to look at the picture.
HEIC or JPG
The iPhone shoots HEIC and other people's computers will not open it. What the format is and how to live with it.