Notes on images
Longer pieces on the questions that sit behind the converters: why files are the size they are, what the settings actually do, and what your photos give away.
Why the same photo is 3 MB as a JPG and 30 MB as a PNG
Save one photo in five formats and the sizes range from about 1 MB to over 30 MB. The reasons are simple arithmetic and a little bit about what compression can and cannot find.
Aspect ratios, and why your photo keeps getting cropped
You upload a group photo and someone at the end has lost an arm. The photo was fine; the shape was wrong. A quick guide to aspect ratios and avoiding surprise crops.
Scanning receipts and documents with your phone: PDF, JPG or PNG?
Upload portals want “a clear scan, under 5 MB, PDF or JPG”. A phone can do that easily, if you know which format suits which kind of document.
Why colours shift when you convert an image
The file converts fine, but the reds look flat and the blues have gone slightly purple. Usually the pixels are not the problem. The instructions for reading them are.
Favicons: the handful of files you actually need
Favicon generators often hand you twenty files. Most sites need five or six. Here is what each one is for, and a few quirks that catch people out.
Resizing images without turning them to mush
Making an image smaller should be the easiest edit there is. Done carelessly it leaves jagged lines and shimmering patterns. Enlarging is harder still.
The location data hiding in your photos
A phone photo can pin your front door to within a few metres. Some apps strip that quietly; many do not. Here is how to tell, and what to do about it.
Why a transparent PNG turns black (or white) when you convert it
You convert a logo and the clean transparent background becomes a black rectangle. The black was there all along. Here is where it hides, and how to keep edges clean.
What the JPEG quality slider actually changes
Quality 80 versus 95 can mean a file twice the size with no visible difference. Knowing what the slider does inside the file makes choosing a number much easier.
DPI is mostly a label: what really decides how big you can print
A print shop asks for 300 DPI and your file says 72. Before you panic, it helps to know that the number is a note about printing, not a property of the picture.
Your iPhone photos are HEIC. When does that actually matter?
HEIC saves your iPhone a lot of space and occasionally causes headaches. Here is where the problems actually come from, and three ways to deal with them.
JPG, PNG, WebP or AVIF? Choose by what is in the picture
Most format advice starts with the formats. It is easier to start with the picture: is it a photo or a drawing, does it need to be see-through, and where is it going?