iPhoto

The iPhoto app was retired many years ago, replaced by the Photos app.

The iPhoto app doesn’t offer watermarking, AFAIK.

The Photos app includes digital markup support, which can be used for watermarking:

Use Markup to write or draw on a photo in Photos on Mac - Apple Support

Notes on Migrating an iPhoto Library from iPhoto - Apple Community

There are also various apps that provide watermarking. Photoshop is one, though very expensive for this use. Acorn is a graphics editing app that I’ve used, and Acorn can watermark and batch watermark. There are dedicated apps a d other image editors, and a search finds iWatermark Pro, and PhotoBulk, Seashore, and others.

PainterPerson wrote:

Ok. So are you able to answer my question using the Photo app, please.

Blue text here in the Apple forums indicates a clickable link.

Click here to learn how to migrate from the older iPhoto app to the Photos app:

Notes on Migrating an iPhoto Library from iPhoto - Apple Community

Click here to learn how to use the markup tool in Photos app:

Use Markup to write or draw on a photo in Photos on Mac - Apple Support

With the markup tool, you can add simple text or graphics to your photos. Including text for watermarking.

You do not even have to import the photos of paintings into the Photos.app to be able to write text onto your Photo, unless you want to organise the photos of your paintings in the Photos Library. You can draw onto any image with Preview with the same markup tools you have in Photos. For example, this callout has been added using the "Tools > Annotate " menu in Preview.

Preview can even take a picture of your written signature and add it as a signature to any image.

The Photos app is included with macOS, and is free.

As was iPhoto, on other versions of macOS.

Click here for how to: Use Photos on your Mac - Apple Support

Please excuse me,

is it possible to use the Photos app for free

How much space can I have?

Thanks

The Photos app is free and comes with the system. To convert your iPhoto library to a Photos library just drag the iPhoto library file onto the Photos application icon in the Dock or in the Applications folder.

Go to the App Store and do a search for "watermark". You'll find quite a few apps, many are free and/or have demo versions to try before purchasing, that can add watermarks to an image file. You might have to export the file to the desktop and run the watermarking app on it there. You an create something like this but with more transparency if desired: