MergePNG

MergePNG is a free browser tool for combining PNG files into one image. You drop in your files, drag them into order or sort them by filename, and pick a horizontal strip, a vertical stack or a grid. Further settings cover matching heights or keeping original sizes, spacing, outer padding, a transparent, white, black or custom background, and a border, and you can export as PNG to keep transparency or as JPG or WebP. Files are decoded and exported on your device rather than sent to a server, and there's no account or watermark. The same site has PNG to PDF, WebP to PNG and a mixed-format Merge Images tool. A Pro plan at $4.99 a month or $39 a year raises WebP batch conversion to 500 files and adds filename rules and presets. It is run by AstraOcean Ltd.