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Fax Guides· Format Guide

How to fax a PNG image

PNG is awkward to fax because of transparency. This guide covers how to flatten, when to convert to JPG first, and when to send PNG directly.

PNG is the awkward middle child of fax formats. It handles transparency — which fax cannot express — and uses lossless compression that is thrown away during fax rasterization. Most of the time, the right move is to flatten the PNG on a white background and send it, or convert to JPG first. This guide covers the decision and the steps.

Common PNG source

Screenshots, logos, signatures, UI captures

Transparency handling

Flattens to black on fax — pre-flatten on white

Convert to JPG?

Only for photos; leave screenshots as PNG

Why PNG needs extra care before faxing

Fax is a 1-bit or 8-bit grayscale medium. It cannot express alpha channels. When a fax service receives a PNG with transparent areas — common in screenshots with rounded window corners, or logos with transparent backgrounds — those regions are rendered as black by the rasterizer. The result is a fax page with unexpected black blobs where the transparent area was.

Most paid fax services (Fax.Plus, eFax, iFax) detect transparency on upload and auto-flatten onto a white background. Free tiers sometimes skip this step. If in doubt, flatten manually before uploading.

Step-by-step: send a PNG safely

  1. 01Inspect the PNG for transparency. On Mac: open in Preview → Tools → Show Inspector. On Windows: open in Paint 3D → the checkered background indicates alpha channel. On phone: if you can see through it to a home-screen icon, it has transparency.
  2. 02If transparent, flatten onto white. Mac Preview: File → Export As → Format: JPG → quality 90 (this flattens automatically). Windows: open in Paint, Select All, Invert Selection, then save as JPG. Phone: use a free app like ImgFlatten, or simply take a screenshot of the rendered PNG on a white webpage.
  3. 03Upload to your fax app. Fax.Plus, iFax, and eFax all accept PNG; flattening at this step ensures the app does not have to guess.
  4. 04Enter recipient fax number with country and area code.
  5. 05Preview the render in the fax app. The preview shows exactly how the recipient will see the page after monochrome rasterization.
  6. 06Tap **Send** and wait for the transmission receipt.

When to keep PNG vs convert to JPG

  • Keep PNG for: screenshots of software UI, logos on solid backgrounds, signatures, text-heavy exports, line art. PNG compression preserves sharp edges better than JPG.
  • Convert to JPG for: photos of people, photos of paper documents captured by camera, anything with a continuous-tone subject. JPG is smaller and fax cannot distinguish the difference after rasterization.
  • PNG-8 vs PNG-24 does not matter for fax — both flatten to a grayscale bitmap.
  • PNG with EXIF metadata is rare; most PNGs do not carry location data. Less to worry about than JPG on privacy.

Checklist

  • Transparency handled — flattened onto white, or confirmed opaque.
  • If source is a photo, converted to JPG first.
  • PNG under 10 MB (screenshots are usually small enough; high-DPI exports can be larger).
  • Recipient fax number with country + area code.

Common mistakes

  • Uploading a PNG screenshot of a dark-mode app. The dark regions render as solid black on fax, losing visible text contrast. Re-capture in light mode.
  • Assuming PNG is always better quality than JPG for fax. For photos, after fax rasterization they are indistinguishable.
  • Forgetting that PNG logos on transparent backgrounds become black blobs on fax.
  • Exporting PNG at 4K resolution hoping for a sharper fax. Fax downsamples to 200 DPI regardless.

Quick answers

Can I fax a PNG directly without flattening?

Usually yes if the PNG is fully opaque. If the PNG has any transparency and you use a paid fax service, it flattens automatically. On free tiers, flatten manually or convert to JPG to be safe.

Will PNG quality be better than JPG on fax?

For photos, no — after fax rasterization, they are indistinguishable. For text and line art, PNG slightly edges out JPG because the sharp edges survive downsampling better.

Why does my PNG logo show as black on the received fax?

The logo has transparent background, and fax rendered the alpha channel as black. Flatten the PNG onto a white background before uploading — or upload a copy that was pre-composited on white.

Should I convert PNG screenshots to JPG before faxing?

No. PNG is usually smaller for screenshots and preserves UI sharpness better. Only convert to JPG if the PNG is huge (over 10 MB) and the source is photographic.

Can I fax a PNG from Windows directly?

Yes. Open the PNG in Windows Photos, click Print, select Fax as the printer (requires the Windows Fax and Scan feature or a paid service-installed print driver). Alternatively, upload to any online fax service via their web interface.

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