Drop BMP bitmaps — one or a folder's worth. Each is written as JPEG at matched quality, with the byte count beside the BMP's, so the saving is a number you can see.
Why a BMP is so large
A BMP stores three or four bytes for every pixel with no compression at all: width × height × 3 bytes, plus a small header. A photograph from a phone camera as BMP runs to tens of megabytes, which is why email, chat apps and upload forms refuse it or choke on it.
The browser decodes the BMP here and the picture is written as JPEG, searched to the lowest quality that still measures visually identical (multi-scale structural similarity of 0.990 or better on a 512-pixel reference). Both byte counts are shown before you save.
Screenshots and drawings
Old screenshots and diagrams are often saved as BMP. For flat colour and text, JPEG softens edges and PNG is usually both sharper and smaller; the converter says when a picture looks drawn. Switch the target to PNG, or to Best per picture to let the measurement choose.