Resizing an image on a Mac without wrecking it
Preview will resize anything in about four clicks. Whether the result looks right depends on three things it does not explain — resampling, the difference between pixels and DPI, and which format you save to.
Someone needs the photo "under 2MB" or "at 1200 pixels wide" or "300 DPI for print", and the file you have is a 6000-pixel photograph from a phone. This is a four-click job on a Mac, and it goes wrong in three specific ways.
The four clicks
Open the image in Preview. Tools → Adjust Size…
You get width, height, resolution, and two checkboxes. Type the width you need, make sure the proportions stay locked, click OK, save. Done.
To resize several at once: select them all in Finder, open them together in Preview, select all the thumbnails in the sidebar, then Adjust Size. It applies to everything selected. This is the single most useful thing in this article and it is the least known.
Understanding the box you just used
Scale proportionally keeps the aspect ratio. Unticking it lets you type a width and a height that do not match the original shape, which stretches the picture. There is essentially no legitimate reason to untick this. If you need a different shape, crop — do not squash.
Resample image is the one that matters. With it ticked, changing the size changes the number of pixels: the image is genuinely rebuilt at the new dimensions. With it unticked, the pixel count stays fixed and you are only changing the print size and the DPI number attached to the file.
Which brings us to the confusion that causes most of the trouble.
Pixels and DPI are not the same thing
A digital image is a grid of pixels. That is all it is. A 3000 × 2000 photograph contains six million pixels whether the metadata says 72 DPI or 300 DPI.
DPI (dots per inch) only means something when the image is printed — it says how densely those pixels get laid down on paper. 3000 pixels at 300 DPI prints ten inches wide. The same 3000 pixels at 72 DPI prints forty-one inches wide and looks terrible.
So when a printer asks for "300 DPI", they are not asking you to change a number. They are asking for enough pixels for the physical size. The arithmetic is: inches × 300 = pixels needed. An A4 page at 300 DPI is roughly 2480 × 3508 pixels.
Changing the DPI field with Resample unticked adds no detail and removes none. It relabels the file. If you had 800 pixels and the job needs 2480, setting the field to 300 does not help anybody.
Downscaling is safe, upscaling is not
Making an image smaller throws pixels away in a controlled manner, and the result generally looks fine or better — downscaling hides noise and minor softness.
Making it larger has to invent pixels that were never captured. Preview interpolates between neighbours, which produces a larger image that is softer than the original, with mushy edges. No amount of sharpening afterwards puts back detail that does not exist.
The practical rule: resize down from the largest original you have. Never resize up, then down, then up again — every resample is lossy in a way that compounds. Keep the original, export copies.
The format decision undoes the work
You can resize perfectly and still ship a bad file.
JPEG is lossy. Every save re-compresses, and re-saving a JPEG repeatedly degrades it even if you change nothing. Good for photographs, bad for anything with text or hard edges, where it produces visible halos.
PNG is lossless and supports transparency. Correct for screenshots, logos, diagrams, anything with flat colour or type. Much larger than JPEG for photographs, sometimes by a factor of ten.
HEIC is what your iPhone shoots — better compression than JPEG at the same quality, poorly supported outside the Apple ecosystem. Convert to JPEG before sending to someone whose setup you do not know.
The common mistake is exporting a screenshot as JPEG to save space. The text goes fuzzy and the file is often not much smaller. Screenshots are PNG.
To change format in Preview: File → Export…, then pick from the Format dropdown. Holding Option while opening that dropdown reveals a few extra formats.
When Preview is not enough
Preview resizes and converts. It does not let you resize part of an image, keep an editable version, undo a change made three saves ago, or apply a repeatable treatment to a batch — scale, crop to a fixed ratio, and add a consistent element.
It also has no concept of layers, which matters the moment you want the original photograph preserved underneath whatever you did to it.
Where Miyu Paint comes in
Miyu Paint is a native image editor for Mac that covers the everyday jobs — open, crop, resize, draw — with free PNG and JPEG export up to 2048 pixels on the longest edge, with no watermark. For screen work, a social graphic or a tidied-up screenshot, that ceiling is usually above what you need.
Full access adds the parts Preview has no answer for: pixel, text, shape, group, mask and adjustment layers, so the composition stays editable; export at full resolution; advanced formats; and printing. The trial is fourteen days and you start it yourself — it does not begin the moment you open the app, and it does not renew into a charge.
Editing happens on your Mac. There is no account, no advertising, and no analytics.