Choose JPEG for a batch of photographs when smaller files and broad compatibility matter more than preserving every pixel. Choose PNG for screenshots, interface graphics, logos, text-heavy images, or artwork that needs transparency or lossless saving. On a Mac, Preview can convert selected images in one window to a chosen format; first decide what the files need to preserve, then export a small representative group and inspect the result before converting the rest.

Decide what the exported images need to keep

Use JPEG when the images are photographs or other continuous-tone pictures, and some compression-related loss is acceptable. JPEG’s core coding system includes lossy compression, and Apple’s Preview export controls let you adjust image quality for JPEG. Lowering the quality can reduce file size, but it also discards image information; there is no universal quality setting that guarantees a particular size or appearance. JPEG.org’s overview of JPEG 1 describes its coding system, and Apple’s Preview conversion guide documents the quality control.

Use PNG when you need pixel values preserved through compression or need transparency, such as a logo placed over different backgrounds. The W3C PNG specification defines PNG as a lossless compressed image format and describes its transparency support. W3C’s PNG specification

A useful batch rule is to group files by intended use, not just by their current extension. A folder of photos and a folder of transparent icons may require different outputs. Converting a JPEG to PNG will not recover detail that was already discarded when the JPEG was made; PNG can preserve the decoded image data without further lossy compression during that export, but it cannot reconstruct the original capture.

Use Preview to convert a selected batch

  1. In Preview, open the images together in one window. If they open in separate windows, choose Preview > Settings > Images and select Open groups of files in the same window, then reopen the group. Apple documents both the setting and the multiple-image window behavior in its guide to displaying several files in Preview.

  2. Show the sidebar if needed with View > Thumbnails, then select the images you want to convert. Keep one output format per batch: selecting JPEG in the export flow applies that choice to the selected images, so separate PNG-needed files into another group.

  3. Choose File > Export, pick JPEG or PNG from Format, and choose a destination and name as prompted. For JPEG, adjust the quality control if it appears. Preview offers format choices including JPEG and PNG, and Apple specifically documents selecting multiple images in the sidebar before following the export steps. Apple’s conversion instructions

  4. Open a few exported files and check the things that matter for your use: edges around text, gradients, transparency against a contrasting background, and whether a JPEG shows visible artifacts. If the preview does not meet the need, export again from the original files with a different format or JPEG quality. Keep originals until you have checked the output.

When Finder Quick Action is the better fit

For a quick conversion from Finder, select image files, open the contextual menu, choose Quick Actions > Convert Image (or More > Convert Image, depending on the menu), then choose a format and size. Apple documents Convert Image as a Finder Quick Action and gives JPEG and PNG as examples. This is a convenient route when its available format and size choices fit the task; use Preview when you want its documented JPEG quality adjustment or want to select a group in one Preview window. Apple’s Finder Quick Actions guide

Neither format is automatically “best” for every mixed batch. If the receiving service or app requires a format, follow that requirement. Otherwise, sort by purpose: JPEG for photo delivery where compact files are useful; PNG for graphics where transparency or exact lossless storage matters. Test the edge cases, then export matching groups from the Mac using Preview or Finder’s Convert Image action.

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