1) I dont know how to write a watermark, like a copyright notice, on a photo.
2) I have a folder of photos which I wish to watermark with the same watermark.
How should I write a watermark on a folder of photographs?
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There are numerous applications with this feature and if you already have image processing software you may have access to this feature.
One standout application that can easily do this is XnConvert. This is a GUI version of
nconvert with facilities for batch processing, including automated watermarking, conversion and metadata editing, so that you also embed a copyright notice too.
XnConvert is essentially free for personal use but it also has a very affordable company licence price. It is available for Windows, MacOS and a variety of Linux distributions.
This can be done in Lightroom very easily and in Photoshop with a few more steps but it is overkill to spend money on an expensive software just for watermarking. Some people even do it from the command line using a shell command and free nconvert utility.
One extremely important thing to pay attention to is to watermark on export. Most people do not watermark their original files, that would not be reversible in case you need one day to make a print or a different watermark.
For this, I think I'd use ImageMagick. It's free and oriented primarily toward batch operations like this. The ImageMagick web site has samples for doing watermarking.
Edit: As @Sean pointed out, it is worth noting that ImageMagick is a command-line based tool, so to use it you need to create the right command line. It may not be the most suitable tool for beginners, especially those who've never used command-line based tools.
1) This is simply a matter of adding your watermark on a separate layer above your photo and any adjustment layers. It can be text or a logo - anything you like. Then simply lower the opacity so the watermark isn't too prominent. You can also change the Blending Mode of the Layer to something like Overlay to help it blend in a little.
2) The simplest way is to record step 1 as an action, then use File > Batch to apply the action to a collection of images. Bear in mind that this doesn't give you any control of the placement of the watermark in every image: if you recorded the action to place the watermark in the centre of the image, it will be in the centre of every image.
Reading the answers, most of them refer to a specific program, and not an overall process, so let me address that.
First, you need to define the style for your watermark, and your copyright notice for that matter.
A copyright notice could be in your webpage as a notice, or on the image itself the IPTC metadata of the image.
A watermark can be a hidden watermark or a visible watermark.
A hidden or invisible watermark is not really a popular way to mark your image. Specialized software is needed to both embed a nonvisible watermark and to read it. And the idea behind it is like an "I got you!" Approach.
So let's focus on a visible watermark.
Here we have probably three options. An intrusive watermark, a small notice, or a signature.
A. The intrusive watermark is one that obstacle the usage of an image by placing a repetitive pattern all over it.
B. A small paragraph of text.
C. A signature is integrated as a part of the image, meant to look good.
Normally for A and C, you prepare a transparent image with only the text you want to overlay opaque or semitransparent.
For A, most likely you need a file with the same proportion of your target images but for the signature alone you can export only the signature.
And if you only want some text (B), some programs let you write it directly in some dialog box.
The next step (or probably the first one) is to back up your files. You do not want to overwrite your files by accident.
Then you need software that allows you to do a batch process.
You can see the different options the other answers provide and need to see specific steps for that program.
You do not need an image editor like Photoshop. This task can be done by some image viewers, for example:
Here is a dialog box on the batch processor. I suggest that you look for specific steps on your specific software googling for example: Irfanview watermark
On the options, you can choose the image you prepared as the watermark, and define the position, the scale, and sometimes additional transparency.
You can automate this using a Photoshop Action.
Basically you want to add a text layer, then set the layer opacity down to level that looks good (test on light and dark images). You can add layer style "bevel and emboss".
This looks to be a pretty good tuturial on the subject:
If you want several files watermarked at once, or a folder full of files, you can use my watermarking program SquiggleMark. (Disclaimer: I am the author of SquiggleMark)
It is free to use.
You can position your watermark, resize it, set its transparency, and watermark a bunch of files all at once. Works on Windows, Mac and Ubuntu.
I am personally doing it like this: http://www.paintshoppro.com/en/pages/watermark-photos/ it might be simpler. You should give it a try. ;)