Lossless and small are usually not available together
Lossless means every pixel value survives the round trip. On a photograph that leaves almost nothing to save, because the JPEG encoder already removed the redundancy a lossless coder feeds on. Expect single-digit percentages, and sometimes a bigger file.
- Lossless Pixels identical, file barely smaller. Right for screenshots, logos, line art, diagrams, and anything with flat colour and hard edges.
- Visually identical Pixels changed, difference invisible at normal viewing size, file often 60 to 90 percent smaller. Right for photographs.
The recipe that works for photographs
- Start from the best original Compress the file straight off the camera, not a copy you already compressed. Lossy passes stack, and each one throws away more.
- Choose WebP At a matching quality setting, WebP usually lands smaller than JPEG, and it keeps transparency. Choose JPEG when the file has to open in older software.
- Set a high quality, then look Use the before and after view at full size. Check skin, sky gradients, and text in the picture. Those three break first.
The quality value drives the browser's encoder. It is not a universal visual score, so the same number behaves differently across browsers and across images. Trust your eyes and the measured byte count.
When PNG is the right answer
PNG stays lossless, which makes it correct for screenshots, UI mock-ups, charts, and artwork with flat colour. On those images the format finds real redundancy and the file drops.
PNG output in this tool is a fresh lossless re-encode of the rendered pixels. Browsers do not expose a portable PNG compression level, so the result depends on the browser. Feed it a photograph and the file often grows. When a candidate comes out larger than the original, Vaultaire discards it rather than offering you a worse file.
For flat-colour images that also need to be small, converting PNG to WebP gives you a lossless option in a denser format.
The lever most people forget
Quality settings are the second lever. Dimensions are the first. A 4,000-pixel-wide photo displayed in a 900-pixel column carries four times more pixels than the screen shows, and every one of them costs bytes.
Halve the width in the image resizer and the pixel count drops to a quarter before the encoder does any work. The result looks sharp at its new size and needs far less aggressive compression to hit a target. When a file-size ceiling refuses to fit, this is nearly always the fix.
Why the download stays locked at first
Before the compressor unlocks a download, it decodes the new file and checks the format, the dimensions, the pixel count, the byte size, the transparency, and the supported metadata. A candidate that fails any check never reaches your disk.
That check runs in your browser, on your machine. The image never travels to a server, so a private photo stays private while you experiment with settings.
Compression quality FAQ
Can you compress an image without losing quality?
Truly lossless compression keeps every pixel value and usually saves little on a photograph. A high-quality lossy re-encode reaches a much smaller file with no difference you can see at normal viewing size. Those are two different promises, so pick one on purpose.
Is WebP better than JPEG for compression?
WebP usually reaches a smaller file than JPEG at a matching quality setting, and it also supports transparency. Every current browser decodes it. Send JPEG when the file must open in old software or a print workflow.
Why did my PNG get bigger?
PNG output is a fresh lossless re-encode, and browsers do not expose a portable PNG compression level. A photograph re-encoded to PNG can easily grow. Vaultaire discards a candidate larger than the original rather than offering it.
Does re-compressing a JPEG twice lose more quality?
Yes. Each lossy pass discards detail the previous pass kept, and the losses accumulate. Compress from the best original you have.
What does the quality number mean?
It controls the browser's encoder and is not a universal visual score. The same number produces different results in different browsers and on different images. Judge the output by looking at it.
What are the file limits?
One still JPEG, PNG, or WebP up to 50 MB, 8,192 px per side, and 16,000,000 pixels in total. Animated files get rejected rather than silently reduced to their first frame.