Batch Watermark allows you to quickly & easily watermark batches of photos. It's free, works inside any modern web browser, and doesn't require any account. Photos are processed on your device and not uploaded to any servers.
Batch Watermark is a spin-off from Bulk Resize Photos (BulkResizePhotos.com), which previously had a very basic watermarking feature.
It became apparent that the watermarking in Bulk Resize Photos wasn't meeting the needs of most users who were interested in watermarking their photos. I surveyed users about why they wanted to watermark their photos, as well as what functionality they were looking for. Batch Watermark is the initial result from that. It's still early days, but I'm looking forward to improving it based on feedback from users, including the Product Hunt community.
Apologies to everyone trying this and finding that applying a watermark image isn't working. I'll have it fixed soon. In the meantime, adding a text watermark should be working.
This issue is now resolved. If you still encounter it, please close all the open tabs with Batch Watermark in them, and then visit the site again. (a reload or two might help as well; sometimes Chrome clings to the cache of a progressive web app a little bit!)
@desmondzik It keeps the format they're already in and tries to be as lossless as possible. Some of it is up to the browser, but generally it should be either lossless or very close to it.
Bulk Resize Photos