Manatee

How to convert a PNG to JPEG on a Mac

· 6 min read

Converting a PNG to JPEG turns a lossless image into a compressed one that is usually far smaller, which is exactly what you want for email attachments, web uploads, and any form or service that refuses PNG files. The quickest way on a Mac is to drop the file on a converter, choose JPEG as the target, and save the new copy. You can also do it with Preview, which is already on your Mac.

PNG and JPEG are built for different jobs

PNG and JPEG are both image formats, but they were designed around opposite priorities. PNG is lossless: it stores every pixel exactly, keeps sharp edges crisp, and supports transparency, which is why it is the natural home for screenshots, logos, diagrams, and anything with text or hard lines. The cost is size. A photograph saved as PNG can be several times larger than it needs to be.

JPEG is lossy. It discards detail the eye is unlikely to miss and, in exchange, compresses photographs into a fraction of the space. It has no transparency, and it is not meant to be edited and re-saved over and over, because each save throws away a little more. For a finished photo that needs to travel light, though, JPEG is hard to beat, and it is the one image format that every device, browser, and website understands.

Why you would convert PNG to JPEG

The usual reason is size. A screenshot tool or an app that exported to PNG can leave you with a 6 or 8 MB file that is awkward to email and slow to upload, when the same picture as a JPEG is a few hundred kilobytes. Some upload forms, print services, and older systems also accept only JPEG. And if the image is a photograph rather than a graphic, JPEG is simply the right format for it; PNG was never the ideal container for continuous-tone images in the first place.

Converting does not repair or improve the picture. It re-compresses what is already there. So the goal is a smaller, more compatible file, not a better-looking one.

Convert a PNG to JPEG on a Mac

Manatee does the conversion in a few seconds and can handle a whole folder at once:

  1. Open Manatee and choose Convert, or drag the PNG straight onto its menu bar icon.
  2. Pick JPEG as the target.
  3. Drop the file, or several files, into the window. Each one appears in the job list with progress.
  4. The new JPEG is written alongside the original. The PNG is left untouched, so you always keep the lossless version.

Everything runs on your Mac; nothing is uploaded and there is no account. Manatee never changes the original file, which matters here, because a JPEG cannot be turned back into the exact PNG you started with.

The built-in route uses Preview, which is already installed:

  1. Double-click the PNG to open it in Preview.
  2. Choose File, then Export.
  3. Set the Format to JPEG and adjust the Quality slider.
  4. Click Save.

Preview is perfectly good for one file. Its limits are that you convert images one at a time, and it silently flattens any transparency onto a white background as it saves. QuickLook, the space-bar preview in Finder, only shows the image; it does not convert it.

Transparency and quality: what actually changes

Two things change when a PNG becomes a JPEG, and both are worth understanding before you delete the original.

Transparency disappears. JPEG has no concept of a see-through pixel, so any transparent area is filled in with a solid color, usually white. If your PNG is a logo meant to sit on a colored background, the JPEG will show a white box around it. That is not a fault in the converter; it is how JPEG works.

The image is re-encoded lossily. Even at high quality, JPEG discards some fine detail, and it is most visible on sharp edges and text, where it can leave faint halos or smudging. On a photograph you will rarely notice at a sensible quality setting. On a screenshot full of small type, you might. This is also why you should convert from the original PNG each time rather than re-saving a JPEG over and over; every save compounds the loss.

When to keep the PNG instead

If the image is a screenshot, a diagram, a logo, or anything with crisp text and hard edges, keep the PNG. Those are exactly the images JPEG handles worst, and the size saving is often small because such graphics compress well as PNG anyway. Keep the PNG too whenever transparency matters, or when the file is a working master you will edit again later; save the JPEG only as the final export for sharing.

A practical habit is to keep the PNG as your archive copy and generate a JPEG whenever something needs to be sent or uploaded. Because Manatee writes the JPEG as a new file next to the original, you get that arrangement without any extra effort.

Questions

Will the JPEG look worse than the PNG?

At a high quality setting, a photograph will look essentially identical. Images with sharp text or hard edges can show faint artifacts, because that is where JPEG’s compression is most visible. If the picture is line art or a screenshot, staying with PNG usually looks cleaner and is not much larger.

What happens to a transparent background?

It is filled with a solid color, normally white, because JPEG cannot store transparency. If you need the background to stay clear, keep the PNG or use a format that supports transparency.

Can I get the PNG back from the JPEG?

Not exactly. You can convert the JPEG back to PNG, but the detail JPEG discarded is gone for good, so the result will not match the original PNG. Always keep the PNG if you might need the lossless version again.

How much smaller will the file be?

For a photograph, often five to ten times smaller. For a simple graphic or screenshot, the difference can be small, and occasionally the JPEG is even larger, which is another sign the image was better off as PNG.