When should I resample an image, and when should I only change its displayed size?

Think of an image as a grid of pixels. Resampling changes the number of pixels in that grid. Changing the displayed size changes how large the image appears in a viewer or layout, without necessarily changing the image file’s pixel dimensions. For example, zooming in to inspect a picture changes its on-screen appearance, not its pixel dimensions.

The exact controls depend on the app. In Preview on Mac, Apple describes changing dimensions with “Resample image” deselected as reducing the image’s dimensions without losing detail, and says selecting it changes pixel data. Those instructions describe Preview’s controls specifically, not every editor’s terminology. [S1]

Start with what the destination requires:

1. If you only want the image to take up more or less space in a document, page, or viewer, adjust its placement size or zoom. This can leave the source image file unchanged, provided you’re changing the layout or view rather than exporting a modified image.
2. If the destination specifies pixel dimensions, or the image file itself must contain a different number of pixels, resample to the required dimensions.
3. If there’s no pixel-size requirement, don’t resample just to make the image look larger or smaller on your screen; adjust its displayed size instead.

For screen-oriented images in Pixelmator Pro, Apple advises using pixel units in its Image Size dialog. That is guidance for that app, not a universal rule for every application or destination. [S2]

Check the control’s label and context. A zoom percentage or a width setting in a document layout may affect only the view or placement. An Image Size, Adjust Size, or pixel-width-and-height dialog may edit image dimensions. Treat the labels as clues rather than guarantees: apps differ, and Apple’s Preview instructions apply specifically to Preview on Mac. [S1]

If the app shows pixel width and height, note them before editing. Also check whether proportional scaling is enabled if you want to preserve the image’s shape. Pixelmator Pro’s Image Size dialog has proportional scaling on by default; don’t assume another app behaves the same way. [S2] If you’re uncertain, work on a copy and inspect its dimensions after saving.

Suppose an image is 2400 × 1600 pixels and you want it to appear smaller in a slide. If the slide editor lets you reduce its placement size, use that and leave the source image’s pixel dimensions alone. If a separate upload requirement says the file must be 1200 × 800 pixels, use an image-size control that resamples to those dimensions, then check the saved copy. These numbers are illustrative, not a recommendation for a particular service.

Reusable check: “I need the image to look [smaller/larger] in [viewer or layout]” means try a display-size control. “The file must be [width] × [height] pixels” means change pixel dimensions and check whether resampling is enabled. If the app’s labels are unclear, don’t guess: check that app’s help or test on a copy.