Choosing the target format
The question is not which format is best but where the file is going. JPG for anything leaving your control — email, forms, print shops, other people's software — because nothing fails to open it. PNG when the image has transparency, text or hard edges, or when it must stay pixel-exact. WebP when the destination is a web page and the 25–35% saving over JPEG is worth having.
Formats that compress harder are not automatically better. AVIF and HEIC produce beautiful small files that a surprising amount of software cannot open at all, and a file nobody can read has no quality at all from the recipient's point of view.
Transparency is the constraint that bites
Only some formats carry an alpha channel. Converting a transparent PNG or WebP to JPG composites the transparent areas onto white, which is invisible until the image is placed on a coloured background and a white box appears behind it.
If you need transparency, the safe targets are PNG and WebP. Note that GIF's transparency is not the same thing: it is a single palette entry flagged as see-through, which is why GIF cut-outs have hard, jagged edges rather than smooth ones.
Converting does not restore quality
A recurring misunderstanding is that saving a JPEG as PNG improves it. It does not: PNG faithfully preserves whatever the JPEG decoded to, artifacts included, in a much larger file. Detail discarded by an earlier lossy save is gone from the file and no conversion recovers it.
The useful direction is the opposite one. Converting from a lossless or modern format to a widely supported one costs something and buys compatibility; converting to a lossless format costs size and buys the ability to edit and re-save without further degradation.
Batches and file names
Drop as many files as you like and they convert in one pass, then download as a single ZIP. The work happens on your own machine through the Canvas API, so a hundred files is a question of how fast your processor is rather than how patient your connection is.
Names are preserved with the extension swapped, which keeps a converted batch matched up with whatever referenced the originals. Nothing is uploaded at any point, which is the practical difference between this and a server-based converter when the files are photographs of documents or anything else you would rather not hand to a stranger.