Drop any mix of pictures — screenshots, photos saved from the web, old BMPs. Every file this browser opens is written as JPEG at matched quality. Transparent files wait for a background colour.
What it opens
Whatever this browser decodes: JPEG, PNG, WebP, GIF, BMP, ICO and AVIF almost everywhere; TIFF on every browser through this site's own TIFF reader (first page — TIFF to JPG lists every page); SVG drawn at its own size with external references removed; and HEIC where the browser has a decoder of its own, which Safari does. The empty converter lists what your browser opens, tested on the spot. A file that does not open stays in the list with the reason.
Each file's format is read from its bytes. A JPEG that only needs a new extension is better handled by the rename mode, which keeps its bytes identical.
One batch, one ZIP
Every file is converted on its own terms: a transparent PNG waits for a background colour, an animated GIF is written as its first frame, a picture already in JPEG is re-encoded only if you keep it in the batch. The ZIP's manifest records what happened to each.