BMP
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PNG
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Tool guide
BMP is an uncompressed raster from the early Windows era: three bytes per pixel means a 1920x1080 screenshot weighs about 6 MB. This tool decodes the file with the browser's own image decoder, paints it on a canvas, and gives you back either a lossless PNG or a 90 % quality JPG. That matters when a scanner, a point-of-sale driver, or some legacy accounting utility produced a BMP and the form, mailbox, or CMS on the other end refuses that extension outright. See also: go the other way and save PNG as BMP, make the PNG lighter as WebP, build a favicon out of that PNG.
Usually 5-20 times smaller, and often more on screenshots and diagrams. BMP stores raw pixels while PNG compresses them losslessly, so the more flat-coloured area the frame contains, the bigger the win. Photographic content compresses far less because there is little repetition — for those, «Relay JPG» is the better button.
PNG. It keeps every pixel, so thin letter strokes and table rules stay crisp. JPG puts visible ringing around characters on that kind of material, and after a couple of re-saves the text starts to smear. Save JPG for photographs, where the gradients are smooth and file size matters more than pixel accuracy.
The tool relies on the browser's decoder, which does not cover every variant of the format. RLE-compressed BMPs, 16-bit variants, and files with a slightly broken header that Paint still tolerates are the usual failures. Also check the extension is genuine: a TIFF renamed to .bmp will not decode.
A classic 24-bit BMP has no transparency at all — it is RGB only. If you happen to have a rare 32-bit BMP with an alpha channel, the outcome depends on the browser; most commonly the alpha is ignored and an opaque background is used. With «Relay JPG» there is never transparency, since the format cannot store it.
No, it handles one file at a time: load, download, press «Reset Station», load the next. That is a minute's work for a dozen scans; for hundreds of files a local batch converter is the sensible choice. The trade-off buys you never having to upload the originals anywhere.
No. The whole conversion happens in your tab: the BMP is read from disk, decoded by the browser, and drawn on a canvas, and the save buttons build the PNG or JPG from that canvas content. No request carrying the image is ever made, so internal scans and paperwork can safely go through it.
Before: scan_0142.bmp, 2480x3508, 24-bit — 26.1 MB, rejected by the mail server as an attachment
After: A 2480x3508 PNG of roughly 1.8 MB, with the text still pixel-exact
Before: photo_old.bmp, 1024x768, 2.3 MB
After: A 1024x768 JPG of about 190 KB at 90 % quality — no visible difference
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