Image file won't open? Causes from wrong extension to data damage

First sort it: broken or just misunderstood?

The decisive first question: does no program open the file, or only a certain one? If another program opens it without trouble (a browser, Preview, a second image program), the file is intact — then it's a compatibility problem (causes 1–3). If everything refuses, data damage comes into play (causes 4–5). This sorting saves the most time.

Cause 1: wrong or misleading file extension

The most common case — and the most harmless. The extension (.jpg, .png) is just a name tag; it says nothing about what's really in the file. Typical traps:

A file is called .jpg but is actually a PNG or a HEIC (happens with renamed images or ones saved from chats).
Someone manually renamed a file and "corrected" the extension without changing the content.
A download broke off and delivered an HTML error page with a .jpg name.

Solution: check the real format. Our image-info tool reads the actual file characteristics, independent of the name — if it shows a clean format, usually renaming to the right extension or opening in a program that handles multiple formats (any browser: drag the file in) is enough.

Cause 2: your program doesn't know the format (yet)

The file is flawless — your program just can't handle the format. Classics:

HEIC from the iPhone on Windows or in older programs. There's a dedicated HEIC guide with all conversion routes.
WebP or AVIF in older software, Office versions, or mail programs. The seven WebP stumbling blocks are covered in WebP not displaying.
RAW formats (.cr3, .nef, .arw) from cameras — these need either the manufacturer software or a program with RAW support.

Solution: get the format into the respective program — usually by converting to JPG or PNG. Standard formats open on every system without exception.

Cause 3: incomplete transfer

The file isn't broken — it's just not all there. Happens with aborted downloads, interrupted cable transfers, or when a cloud file exists only as a placeholder and hasn't been downloaded yet ("online only"). Telltale sign: the file is conspicuously small (a few KB) or exactly 0 bytes.

Solution: check the file size in the file manager. At 0 bytes or suspiciously little: re-download or re-transfer. With cloud services, explicitly "download to device" before opening it.

Cause 4: real data damage

Rarer, but real: the file is partly destroyed — by a defective memory card, a problem while saving (dead battery during the write), a file-system error. Typical picture: the upper half appears, the rest is gray, a colored bar, or pixel noise.

Solution — realistically framed:

Another source first: does the image still exist on the camera/memory card, in the cloud, in the chat history, in the trash? The undamaged copy beats any repair attempt.
Spare the memory card: if multiple images are affected and the card is the cause — don't write to the card further and read it out with photo-recovery software.
Adjust expectations: a partly displayed JPG can sometimes be saved so at least the intact part is preserved — but the destroyed data is gone. "Repair" here means rescuing what's still there, not magic. Beware of websites that promise "100% repair" and demand you upload sensitive photos.

Cause 5: not an image, but something else

Sometimes the supposed image file isn't one at all: a download disguised as an image, a wrongly named text file, or — with unexpected attachments from strangers — potentially something unwanted. Warning sign: a .jpg file that's actually called .jpg.exe or .jpg.scr (an executable in an image costume). Don't open such attachments — that's a known trick to disguise malware as a harmless photo. When in doubt, check back with the sender through another channel.

The troubleshooting as a short procedure

Does another program open the file? → If yes: compatibility (causes 1–2).
Check the file size — 0 bytes or tiny? → incomplete transfer (cause 3).
Check the real format with the image-info tool → correct extension/conversion.
Partly visible with corruption? → data damage, first look for an undamaged copy (cause 4).
Double extension .jpg.exe? → don't open (cause 5).

In the vast majority of cases the search ends at step 1 to 3 — the file was never broken, just mislabeled or in the wrong program.

Sources

MDN — Image file types on the web · Apple — Media formats of iPhone and iPad.