Compressing images

The core job of imgsqz is making images smaller. This guide covers the quality knob, lossless modes, and tips for squeezing each format.

The quality knob

-q, --quality takes an integer from 1 to 100. Lower means smaller files and more compression artifacts; higher means larger files and better fidelity. The default is 80.

# Aggressive — small files, visible artifacts at close inspection
imgsqz photo.jpg -q 50

# Default — a strong balance for most web imagery
imgsqz photo.jpg -q 80

# High fidelity — larger files, near-original quality
imgsqz photo.jpg -q 92

Finding the right number

Quality scales are per format — webp -q 75 is not the same perceptual level as avif -q 75. Start at the defaults below and adjust by ±10 while eyeballing the result.

Recommended starting points

Goal	Format	Quality
Photographic web images	webp	75
Smallest modern format	avif	50–55
Maximum compatibility	jpg	80
Screenshots / UI / text	png (lossless) or webp --lossless	—
Flat-color graphics, logos	png	80

Compress without converting

To shrink a JPG and keep it a JPG, just set the format to match (or rely on the default for JPG sources):

# Re-encode a JPG at quality 75 (mozjpeg makes it smaller than most originals)
imgsqz big.jpg -f jpg -q 75 --no-timestamp -o big.jpg

For PNG-to-PNG, lower quality enables palette quantization for big wins on graphics:

imgsqz logo.png -f png -q 70

Lossless compression

--lossless switches WebP, AVIF and PNG into a mode that reconstructs every pixel exactly — ideal for screenshots, diagrams, and anything with sharp text.

# Lossless WebP — pixel-perfect, still smaller than PNG for many images
imgsqz screenshot.png -f webp --lossless

# Lossless AVIF
imgsqz diagram.png -f avif --lossless

Note

In lossless mode the --quality value no longer trades fidelity (there are no artifacts) — the output is exact. For PNG, --lossless also disables palette quantization, preserving full color depth.

Grayscale for extra savings

If color isn’t needed, --grayscale drops the chroma channels and shrinks the file further:

imgsqz scan.jpg --grayscale -q 80

Great for document scans, line art, and black-and-white photography.

Reading the savings report

imgsqz prints the before/after for every file:

photo.jpg
  → photo_20260611_132517.jpg
  3.10 MB → 488.21 KB (↓ 84.6%)

• ↓ 84.6% — the output is 84.6% smaller than the input.
• ↑ 12.0% — the output is 12% larger (can happen when re-encoding an already-optimized file, or with lossless modes). If you see this, try a lower quality, a different format, or leave the original as-is.

A quick comparison workflow

Produce several candidates and compare sizes, then keep the winner:

bash / zsh

for q in 50 65 80; do
  imgsqz photo.jpg -f webp -q $q -o "out/photo-q$q.webp"
done
ls -lhS out/

PowerShell

foreach ($q in 50,65,80) {
  imgsqz photo.jpg -f webp -q $q -o "out/photo-q$q.webp"
}
Get-ChildItem out\ | Sort-Object Length

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Next: Converting formats.