To make README screenshots consistent, choose one target width and aspect ratio, capture each screen using the same window or selection framing, then use Preview to crop and resize the images to matching pixel dimensions. macOS’s built-in Screenshot controls offer window and manually selected-area capture, but Apple’s documented controls do not provide a numeric width-and-height entry for the capture frame. Use Preview for precise final dimensions and check that resizing has not distorted interface text.

Choose a repeatable capture frame

Decide what the README image needs to explain: a full app window, a specific interface area, or a sequence of related screens. For a series, keep the subject and framing consistent. Window capture is convenient for a whole app window; a selected area is useful when window chrome or surrounding desktop would distract. macOS supports both methods, as well as full-screen capture. Apple’s Mac screenshot guide

If you capture a selected area with Shift-Command-4, drag over the content and release to take the screenshot. While dragging, hold Space to move the selection without changing its size. That helps place a frame similarly from one capture to the next, but Apple documents no control for entering exact pixel dimensions. Treat manual framing as a starting point, not proof that two files have identical dimensions.

For window capture, open the target window and press Shift-Command-4, then Space, then click the window. Hold Option as you click to omit its shadow. This gives you a straightforward window-oriented image, though different window sizes can produce different output dimensions. Before taking a series, set the app window to a consistent size and keep its visible content arranged similarly. The sizing step is a practical workflow choice; the screenshot tool itself does not lock window sizes.

Set up Screenshot options on Mac

Press Shift-Command-5 to open Screenshot. Choose the capture mode, then open Options to set a timer, choose a save location, control the floating thumbnail, and decide whether to show the pointer. A short timer can give you time to place the pointer away from important text or prepare a menu before capture. Keep the pointer setting the same throughout a series. Apple documents these Screenshot options.

Use one save folder so the source images are easy to find. If README compatibility across repository viewers matters, select SDR (Most Compatible) when the Screenshot app offers the format choice; Apple says this option uses PNG, while HDR uses HEIF on supported Mac models with macOS Tahoe 26 or later. Do not assume that every Mac or older macOS version shows this format menu.

Crop and resize in Preview

Open a screenshot in Preview. To crop, show the Markup toolbar, choose Rectangular Selection, drag around the area to keep, then click Crop. If available in the Crop Inspector, you can inspect the selected content’s dimensions. Apply the same crop boundaries or aspect ratio to every image in a set so the visual framing matches. Apple’s Preview guide explains cropping and dimension inspection.

Next, choose Tools > Adjust Size. Enter the desired width and height, and keep Scale proportionally selected when preserving the original proportions matters. Preview may make one dimension smaller than the requested size to preserve those proportions. Forcing unrelated width and height values can stretch the interface; if the image’s proportions do not match your target, crop to the target shape first, then resize. Save a copy if you want to retain the uncropped original.

Preview can adjust multiple images together when they are displayed in one window and selected in its sidebar. This can help apply common dimensions, but check each image afterward: a shared resize does not make their crops or subjects match automatically. Apple describes batch resizing in Preview.

Verify the images in your README

Compare the final files in Preview: check their pixel dimensions, aspect ratio, crop boundaries, legibility, and pointer visibility. Then add them to the repository and reference them with relative paths in Markdown, for example ![Settings screen](images/settings.png). GitHub documents relative image paths for rendered repository content, including README files. GitHub Docs: About the repository README file.

A useful repeatable sequence is: set the app window, set Screenshot options once, capture the series, crop all images to one aspect ratio, resize proportionally to one target width, and verify the resulting dimensions. The built-in tools make consistent results practical, but exact capture-frame dimensions require checking and adjusting the resulting image in Preview.

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