Picking an output format, and what the quality slider is doing
Format conversion is where most of the file-size wins on this site actually come from, and where the most confident wrong answers get given. The useful framing is not “which format is best” but where is this image going.
Destination decides
A file bound for a web page you control can be WebP or AVIF, because you know what will read it. A file bound for a colleague, an upload form, a print shop or a piece of software you have never heard of should be JPEG — thirty years old, worse compression, and it opens absolutely everywhere. A file with transparency or crisp text has different needs again: lossy compression is built on the assumption that neighbouring pixels resemble each other, which is exactly wrong for a logo on a transparent background or a screenshot full of type.
Getting this decision right is worth more than any amount of quality-slider fiddling. Sending a 4 MB PNG photograph where a 300 KB JPEG would do is a larger mistake than any quality setting can be.
The quality slider is per-format
Each format has its own scale, and the numbers do not transfer between them. The estimated output size updates as you move the slider precisely so that you do not have to carry a number over from another tool and hope. For photographs, the point where the estimate stops falling steeply is usually close to the point where further reduction starts costing visible quality.
PNG has no quality slider at all, because it is lossless — every pixel is stored exactly. A PNG of a photograph is not higher quality than a good JPEG in any way you can see; it is simply five to ten times larger.
Converting a batch
The queue accepts mixed input formats — JPEG, PNG, WebP, GIF and BMP everywhere, plus HEIC and AVIF where the browser can decode them — and normalises everything to the single output format you choose. That is the quickest way to deal with a folder of phone photos in HEIC that some other piece of software refuses to open.
Everything is encoded on your device, one file at a time, and the ZIP is assembled in your browser as well. Nothing is uploaded at any point, which also means there is no per-file size ceiling other than your own memory.