A slider can be labelled. It cannot, in the width of a 300-pixel panel, explain why it is there. Each of these notes began as a question somebody asked after a conversion had already run and could not be taken back: where did the see-through part go, why does this file have a different extension from the one I asked for, why will the phone open it and the desktop refuse.
They are notes, not tutorials. None of them walks you through this page — there is a numbered list on the front of the site for that, and it is five lines long because the job is five lines long. What these do is name the bargain the format is striking on your behalf, so that what is in front of you is a decision rather than a default somebody else picked.
What HEIC is, and the ways out of it
A container borrowed from video, a codec that halves the file, and the licence that keeps most of Windows from reading either. Start here if a folder of holiday photographs will not open on a laptop.
JPG and JPEG are one format
The answer is in the first line and takes four words. The rest is the three-letter filename limit that produced a question people were still asking thirty years after the limit went away.
Transparency has to become a colour
JPEG has nowhere to put an alpha channel, so every transparent pixel is painted before it is encoded. How to choose what it is painted with, and how to tell when the right answer is to keep the PNG.
What is deliberately not written down here
There is no note on getting a photograph under a stated number of kilobytes. That is a measurement loop — encode, weigh, adjust, encode again — and this converter does not run one, so a piece explaining how to do it here would be explaining something that is not on the page.
There is no note on chroma subsampling either, and the reason is the same shape. The setting is real, it matters most on red text and hard edges, and on this site the browser’s own encoder picks it without asking. Writing four hundred words about a switch nobody can reach would be worse than the silence.
And there is nothing here about editing a photograph — cropping it, lightening it, taking something out of it. None of that happens on this domain, and a converter that explains how to crop is a converter that has started to drift.
The converter itself is at the front of the site, and the iPhone case has its own page with the codec warmed up in advance.