JPG to ICO
An ICO wraps a PNG payload in an icon container that Windows and all browsers accept. Starting from a JPG works, with one caveat worth knowing: JPG has no transparency, so the icon will have a solid rectangular background. For a favicon that sits on a coloured tab bar, start from a PNG with alpha instead.
Icons are displayed at 16–48 pixels. Detail that reads clearly at full size disappears entirely — favicons need to be simple shapes with strong contrast, not shrunken logos.
What Each Format Is For
JPG. JPG is lossy and has no transparency at all. Its discrete-cosine compression is built for photographs — smooth gradients, natural detail — and visibly degrades sharp edges and text into ringing artefacts. Every save re-encodes and loses a little more, so JPG is a delivery format, not a working one.
ICO. ICO is a container rather than a codec: it wraps one or more images, at different sizes, in a single file that Windows and browsers read as an icon.
| JPG | ICO | |
|---|---|---|
| Compression | Lossy | Lossless available |
| Transparency | No | Yes |
Transparency Survives
Both formats carry an alpha channel, so transparent regions convert without being flattened onto a background. That makes this a safe conversion for logos, icons and cut-out product photography.
The Conversion Runs in Your Browser
The file is decoded and re-encoded locally through a canvas. Nothing is uploaded, nothing is stored, and the conversion works with the network disconnected. For anything containing personal data — identity documents, medical images, private photographs — that is a meaningfully different risk profile from any server-side converter.
The practical limits are your device's memory on very large images, and the browser's own codec support: it can only encode formats it ships an encoder for, and the tool tells you when it cannot.
Quality Settings
ICO can encode losslessly, so a conversion from a lossless source is exact — every pixel is preserved and re-encoding again later costs nothing.
One-Way Doors
Re-encoding never recovers information. If the target is lossy, keep the original: a ICO converted back to JPG has the same artefacts as the ICO, in a larger file. Treat converted output as a delivery copy and archive the source.
Sizing Before Compressing
Resizing beats compressing. Halving an image's dimensions removes 75% of its pixels, which no quality setting can match, and a 4000px photo displayed at 800px is wasting 96% of what it downloads.
| Displayed at | Serve (1×) | Serve (2× for retina) |
|---|---|---|
| 400px | 400px | 800px |
| 800px | 800px | 1600px |
| 1200px | 1200px | 2400px |
Quality Settings That Hold Up
| Setting | Looks | Use for |
|---|---|---|
| 95–100 | Indistinguishable, large | Masters and print |
| 80–90 | Very good | Hero images, product photography |
| 70–80 | Good | Most web content |
| 50–70 | Visible artefacts on detail | Thumbnails |
| Below 50 | Obvious | Nothing |
Everything Runs Locally
Conversion and compression here use a canvas in your own tab. No file is uploaded, nothing is stored, and the work continues offline once the page has loaded. That is worth knowing for anything containing faces, documents or unreleased work — most online image tools upload by necessity, because they process server-side.
Getting the Rest of the Win
- Serve modern formats with a fallback.
- Always set
widthandheight. Without them the page reflows on load, which
- Lazy-load below the fold — and never the hero, since
loading="lazy"on it delays
- Strip metadata. EXIF can carry GPS coordinates and adds weight to every copy.