Watermarking images is a tedious task. Be 10 or 10000, it being manual and the task might come off as boring work to do.
This free tool will help you do it all at once for no cost.
Software Name – IrfanView
Software Download link - https://www.irfanview.com/
Plugin Download link - https://www.irfanview.net/plugins.htm (Optional)

Steps

1. Download and Install the Plugins (Optional)
2. Once opened, go to File > Batch Conversion/Rename. A new window will open.
3. On the right Please go to the folders with the images and click “Add all”.
4. Chose the Output folder (where you want the processed images to go) on the left.
5. Now, Click the advanced option on the top. A new window will pop up.
6. Click the “settings” option next to the “Add watermark image” section. A new window will pop up.
7. Here you will have the option to add the logo and also define logo size, Position, and Opacity. Once done click ok.
8. Finally, click, Start batch. The conversion will start and you will be notified once done.
9. You can also rename the output images with the name pattern on the top.

Note: There is no restriction to how many images you can process at a time. I have personally tested it for 10000 images.

Hope this helps. If you have any trouble using this application, feel free to drop an email, will be more than happy to help you out!