Bulk Operations·4 min read

Choosing One Output Format for a Batch of Images

Legacy image folders are archaeology: PNG screenshots from one designer, JPG camera files from another, a few HEIC photos from someone's phone, a TIFF scan nobody can open. A CMS migration or a new CDN usually wants all of it in one format. Choosing that format for a whole batch is a different decision from converting one file, because the choice has to suit the worst file in the folder, not the typical one. This guide walks through the choice and the settings that go with it.

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Choosing one output format for a batch of legacy files

The worst file decides.

Start by sorting the folder in your head into photos, graphics with sharp edges, and anything with transparency. Photos compress well in any lossy format. Screenshots with small text blur first. Logos and cutouts need a format that keeps transparency, or a background colour you are happy with.

If every group is present, the choice narrows quickly: WebP or AVIF keep transparency and compress photos well, PNG keeps every pixel but makes photos heavy, and JPG only works if nothing needs to be see-through. GIF, BMP and TIFF exist for old systems and print, not for the web.

What happens to each file in the queue

Decode, draw, encode.

The Bulk Image Converter decodes every input in your browser. JPG, PNG, WebP, GIF, BMP, ICO and SVG open natively, AVIF opens where the browser supports it, and TIFF and HEIC are decoded by the tool itself, so they take a moment longer. Animated GIFs contribute their first frame, and SVG files need a width and height or a viewBox.

Each image is drawn on a canvas, scaled down first if you picked a longest side, and saved in the target format. Formats without transparency get the background colour you chose. The queue shows every file's size before and after, and duplicate names such as photo.png and photo.jpg become photo.webp and photo-2.webp so nothing is overwritten in the ZIP.

WebP or AVIF for a whole library

The web defaults.

WebP is the safe default for a website: every current browser displays it, it keeps transparency, and it is usually much smaller than PNG and noticeably smaller than JPG. AVIF often goes smaller again at the same visual quality, which matters on image-heavy pages. It is slower to encode, not every browser can save it, and some desktop apps still cannot open it.

A practical split: convert the web library to AVIF where your browser offers it, or WebP where it does not, and keep one JPG copy of the set for email, print services and partners on older software. The AVIF button is greyed out in browsers that cannot encode it, so you always know which path you are on.

A worked queue: forty PNGs to WebP at 92

Settings, run, check.

Drop forty PNG screenshots and product cutouts, choose WebP under Convert to, and leave the quality at 92. Set a longest side of 1920 px so the 4K screenshots come down to a sensible size, and keep EXIF off. For names, use {name} to keep the originals.

Click Convert to WebP. Input: dashboard-home.png at 1.8 MB and 3840 by 2160 comes out as dashboard-home.webp at 1920 by 1080 and a fraction of the size, with transparency intact on the cutouts. Check the smallest text in two screenshots at full zoom before downloading converted-webp.zip. If the text looks soft, run those files again at 95.

Transparency, colours and metadata

The settings people skip.

Converting to JPG or BMP fills every transparent pixel with the background colour. White is the default, but a brand colour or the page background of your site often looks better. For PNG and GIF output, Reduce colours cuts the palette to 256, 128, 64, 32 or 16 colours with dithering, which can shrink flat graphics a lot and ruins photos, so test on one file first.

EXIF data such as the camera, date taken and GPS location is removed by default, which is what you want before publishing. Switch on Keep EXIF data for JPG, PNG and WebP output when the photos are going into an archive or a photo library that sorts by date.

Batch conversion mistakes that surface weeks later

These are the problems that only show up after the files are live.

Converting logos to JPG and discovering white boxes on a dark footer.
Converting screenshots at a low quality to save bytes, then fielding complaints about blurry text in the help centre.
Re-running lossy conversions on top of each other during repeated migration attempts instead of always converting from originals.
Leaving EXIF on for public photos and publishing home GPS coordinates.
Shipping AVIF only, then finding a partner's email tool cannot show it.

Bulk Image Converter beside the single-pair tools

The PNG to WebP Converter, JPG to WebP Converter, HEIC to JPG Converter and AVIF to JPG Converter handle one known pair, and they are the right link to send a colleague who asks how to convert one file. The Bulk Image Converter earns its place when the folder is large or mixed and the answer must be uniform. If size still matters after the format change, the Bulk Image Compressor can take the output further down with a per-file target, and the Bulk File Renamer can apply a naming scheme afterwards.