Changing an image’s file type sounds like renaming it. It is not. Renaming photo.png to photo.jpg produces a broken file with a misleading name — the bytes inside are still PNG, and anything strict about formats will refuse to open it.

A real conversion decodes the image back to raw pixels and re-encodes it with a different codec. That round trip is where both the savings and the costs come from.

The conversions worth making

JPEG → WebP. The everyday win. Roughly a third smaller at matching quality, and every browser released in the last several years reads it. If you convert nothing else, convert this.

PNG photograph → JPEG or WebP. PNG is lossless, which is excellent for logos and catastrophic for photographs — a photo saved as PNG is often five to ten times larger than it needs to be. This single conversion produces the most dramatic numbers you will see.

Anything → AVIF, when you have the time. Smaller again than WebP, at the cost of a noticeably slower encode. Worth it for images that will be served millions of times; rarely worth it for a one-off.

The ones to think twice about

WebP or AVIF → JPEG. Going backwards costs quality and gains you nothing but compatibility with software old enough to need it. Do it only when something genuinely cannot read the modern format.

Photograph → PNG. You will not recover the detail a lossy format already discarded, and the file will balloon. Converting a JPEG to PNG gives you a larger file containing exactly the same visible flaws.

Repeated round trips. Every lossy re-encode compounds on the last. JPEG to WebP to JPEG to WebP will visibly degrade, even at high quality settings each time. Convert from your original whenever you still have it.

Two things that break in conversion

Transparency. PNG, WebP and AVIF support an alpha channel. JPEG does not. Convert a transparent logo to JPEG and the transparent areas become solid — usually white, sometimes black, and you will not always notice until it is on a coloured background.

Animation. Animated GIF and animated WebP hold multiple frames. JPEG holds one. Convert an animation to JPEG and you keep the first frame and lose the rest.

Converting a folder at once

Converting files individually is fine for three and unbearable for three hundred. In SqueezeImages you set the output format once and drop in the whole folder — every image inside is converted and compressed with the same settings, and subfolders are either preserved or flattened, whichever you prefer.

Two habits worth keeping:

  • Convert into a new folder. Originals are never overwritten unless you explicitly ask, and you want that safety net the first time you try a new format.
  • Check one file of each type before committing. A folder of photographs and a folder of screenshots want different answers.

So which format should you pick?

For most web work: WebP. It handles photographs and flat graphics, keeps transparency, and is broadly supported. Stay with PNG for screenshots and line art where text has to stay crisp, and reach for AVIF when the file will be served often enough to repay the encode time.

There is a fuller comparison in why WebP and AVIF beat JPEG for the web, and a side-by-side table in the format guide on the home page.

Once you have chosen, the guide to compressing without losing quality covers how hard to push. SqueezeImages converts and compresses in the same pass, offline, for free — download it here.