Someone sends you "photo.jpg" and it's actually a PNG. Or you need to know if an image is big enough to print. Or a website rejects your upload and you need to figure out why.
Checking an image's specs takes seconds once you know where to look. Here's every method I use.
The fastest: file properties
On Windows, right-click the file, Properties, Details tab. Format, dimensions, file size, all there.
On Mac, right-click, Get Info. Or open in Preview and hit Command+I. Same information, slightly prettier.
This handles 90 percent of cases. Format, pixel dimensions, file size. Done.
In the browser
Right-click any image on a webpage and pick "Open image in new tab." The tab title often shows dimensions, like "photo.jpg (1600x900)."
For the full picture, right-click, Inspect, and check the Network tab. It shows the actual file served, its size, and its format. Useful when a site serves WebP but the URL says .jpg (happens constantly).
Why the extension lies
The file extension is a suggestion, not a fact. I've seen ".jpg" files that were PNGs, ".png" files that were WebPs, and files with no extension at all.
The properties panel reads the actual file, not the name. When something behaves weirdly (a program rejects a "JPG"), check the real format. Nine times out of ten, the extension was wrong.
What the numbers tell you
Dimensions tell you what the image can do. 4000 pixels wide prints well and crops freely. 800 pixels wide is screen-only. Below 500, it's a thumbnail.
File size tells you about efficiency. A 4000-pixel photo at 200 KB is heavily compressed (check for damage). The same dimensions at 8 MB is uncompressed or barely compressed (shrink it for the web).
Aspect ratio (width divided by height) tells you the shape. 1.78 is 16:9 widescreen. 1.0 is square. Useful when a platform demands a specific ratio.
Practical tip: when a website rejects your upload, check three things in order: format (does the site accept it?), file size (under the limit?), dimensions (meets the minimum?). One of those three is the problem basically every time.
Checking inside a converter
Upload any image to Jiggle X Wiggle's free converter and it shows you the format, dimensions, and size before you convert. Handy when you're already there to fix whatever you found.
Five seconds of checking saves twenty minutes of guessing. Look before you upload.