To balance image quality and file size on a Mac, first match the image’s pixel dimensions to how it will appear online, then export it in a suitable format and compare the result at its intended display size. Preview handles resizing and JPEG quality; Photos offers additional export controls for images in a library. Keep an untouched original so you can make a different version if the first export looks soft, blocky, or loses transparency.

1. Choose the dimensions before compressing

Find the largest size the image needs to appear on the page or in the message. For example, if a site displays an image at about 900 pixels wide, an original that is several thousand pixels wide may be larger than necessary. For a page that adapts to different screens, ask the site owner or content system whether it creates responsive image sizes; a single exported file may not be the only size the site can use. web.dev explains how browsers select image candidates for different display widths and pixel densities in responsive setups. Image performance

Open a copy in Preview and choose Tools > Adjust Size. Set the desired width and height, and leave Scale proportionally selected to avoid stretching. With Resample image selected, changing pixel dimensions changes the image’s pixel data. Preview shows the resulting size estimate at the bottom of the dialog. Apple also documents batch resizing by opening images in one window, selecting them in the sidebar, and choosing Tools > Adjust Size. Crop, resize, or rotate an image in Preview on Mac

Do not confuse pixel dimensions with resolution measured in pixels per inch. For an on-screen image, the width and height in pixels determine how many image pixels are available; changing a print-resolution value alone does not necessarily make a web image fit its display slot. If you need to reduce the actual pixel dimensions, enter smaller width and height values with resampling enabled.

2. Pick a format that suits the image

Use JPEG for photographs when broad compatibility matters and transparency is not required. JPEG uses lossy compression, so reducing its quality can make the file smaller but can also introduce visible artifacts. Preview supports exporting to JPEG and adjusting its quality in the export dialog. Convert image file types using Preview on Mac

PNG is often a sensible choice for screenshots, diagrams, and images with sharp text or transparent areas. Lossless formats preserve image data, while lossy compression is more likely to show flaws around crisp edges and high-contrast text. Image performance

WebP and AVIF can reduce file size in some web workflows, but Preview’s documented export list does not include them. Do not assume the Preview Format menu can create either format. If your website specifically requires one, use a separate tool or the site’s image workflow and check what formats it accepts. Modern formats can offer better compression, but the actual savings depend on the image and export settings. Image performance

3. Export from Preview or Photos

In Preview, choose File > Export, choose JPEG, and adjust the Quality control. Give the export a new name or save it in a separate location so you retain the source. Exporting as JPEG does not preserve transparency, so avoid it for graphics that need a transparent background. Apple documents quality adjustment for JPEG and JPEG-2000 exports. Convert image file types using Preview on Mac

If the photo is in the Photos library, select it and choose File > Export > Export [number] Photos. Photos lets you choose a file type and, for supported options, image size and JPEG quality. The available choices vary by format. Choose Export Unmodified Original when you need the original file rather than a new version with export settings. Export photos, videos, slideshows, and memories on Mac

4. Compare the result and adjust one thing at a time

Open the exported file and inspect it at the size people will see it. Look for blurred fine detail, blocky areas, halos around text, banding in smooth gradients, and missing transparency. Also check the exported pixel dimensions and file size in Finder’s Get Info window. Compare the output against the original, not just the number on the quality slider: there is no universal compression setting that suits every image, and web.dev recommends trying compression levels and checking that the result meets the quality standard. Image performance

If the file is still too large, reduce excess pixel dimensions first if they exceed the intended display needs. If its dimensions are already appropriate, try a modestly lower JPEG quality and inspect again. For text or line art, use a lossless format where practical instead of pushing lossy compression until edges visibly deteriorate. Stop when the next reduction creates a visible defect, and keep the original available for another export.

A practical rule for the handoff

Before uploading, confirm three things: the site accepts the chosen format, the pixel dimensions suit its display, and the exported image still looks clear at that size. Preview provides useful Mac controls for ordinary resize and JPEG export tasks; it does not offer every modern web format or automatically select the ideal settings for a particular site. The final choice depends on the image content and the destination’s requirements.

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