How I built a bulk image converter that converts multiple images in your browser.

Motivation

Most bulk image converters online require uploading files to a server. That raises privacy concerns, especially for sensitive images. Bandwidth costs add up for large batches and free services often have ads or limit file counts.

Bulk Image Converter sidesteps all of that by processing images entirely in the browser using the HTML Canvas API.

Features

• Drag-and-drop or click to upload multiple images at once.
• Batch conversion to WebP, JPEG, PNG, or AVIF.
• Quality control with a slider for lossy formats (WebP, JPEG, AVIF).
• ZIP download of all converted images in one click.
• Per-file status tracking (pending, converting, done, error).

How It Works

1. Upload images — drag-and-drop or click to browse.
2. Select a format — select from WebP (default), JPEG, PNG, or AVIF.
3. Adjust quality — a slider appears for lossy formats (PNG is always lossless).
4. Convert & download — all images are converted in parallel and archived into a single ZIP file.

Technical Decisions

Canvas API

Each image is loaded into an HTMLImageElement, drawn to a HTMLCanvasElement, and exported via canvas.toBlob() with the target MIME type and quality.

JSZip

The converted blobs are bundled into a ZIP file using JSZip. The ZIP is generated in memory and downloaded via a temporary <a> element with URL.createObjectURL().

Try It

You can try the Bulk Image Converter or check out the source code.