Where JPG favicon sources come from
People rarely design a favicon as a JPG on purpose. The JPG usually arrives sideways: a logo pulled from a supplier website, a profile photo used as the icon for a personal site, a scan of a printed logo, or a client who only has the version that was emailed around years ago. JPEG stores no transparency and applies lossy compression in 8 by 8 pixel blocks, and both facts shape what you can expect from the icon.
What happens to a JPG on the way to ICO
The converter decodes the JPG in the browser, draws it onto a square canvas at 16, 32, 48, 64 and 256 pixels, encodes each square as PNG and packs the five images into one .ico file. The PNG step is lossless, so no new compression damage is added, but the existing JPEG artifacts come along. Blocky halos around edges are invisible in a 1200 pixel photo and hardly visible at 256, but at 16 pixels the scaler averages those halos into the surrounding colour, which makes a logo edge look dirty or slightly the wrong shade.
Because JPEG has no alpha channel, every entry in the output is a fully opaque square. A white logo background becomes a white square in the browser tab, which looks fine on light themes and like a sticker on dark ones. This is the single biggest difference from a PNG source.
Preparing the image first
Three small steps make a noticeable difference. First, crop to a square. The canvas stretches the source to fit each square, so a 1600 by 900 photo will come out squeezed horizontally. The crop tool has a 1:1 preset. Second, decide about the background. If you want the logo to float over the tab, run the JPG through the background remover, download the resulting transparent PNG and convert that instead. Third, if the JPG is small, say 100 pixels, accept that the 256 pixel entry will be an upscaled and soft version of it. There is no way to recover detail that the original never had.
A worked example with a profile photo
Suppose the source is a 1080 by 1080 headshot, 210 KB as a JPG at typical camera quality. Photographic content does not compress well as PNG, so the 256 pixel entry alone usually lands between 90 and 150 KB and the whole ICO around 100 to 170 KB. Nobody notices this on a normal connection, but it is ten times larger than the ICO for a flat logo. A simple two colour mark of the same dimensions typically produces a 20 to 40 KB icon. If size matters, use a plain graphic instead of a photo.
How the icon looks in practice
Photos as favicons work best when the subject fills the frame and has strong contrast: a face against a plain wall, a single object on a light table. Group shots, landscapes and anything with fine texture become an unrecognisable smudge at tab size. Check the 16 pixel version by zooming your browser out; if you cannot tell what it is, simplify. The larger sizes in the same file, 48 and 256, will still look good on Windows shortcuts and in bookmark managers, so the file is not wasted even if the tiniest entry is weak.
Limits to keep in mind
The conversion is done entirely on your device, so there is nothing to upload and no size cap other than your device memory. CMYK JPEGs from print workflows may decode with shifted colours in some browsers; export an RGB copy first. Each JPG produces its own .ico download, there is no bulk archive. For a complete set including PNG icons for phones and tablets, use the favicon generator.