Depending on the software you're using, it can be easy or hard.
If you've got Adobe AfterEffects, it's easy:
Open AFX, Add your current video clip to the timeline, add a single image with your watermark to the B video channel and adjust it's display duration to be the length of the entire clip.
Adjust the transparency of the video B layer until you get what you like and render the work area. It would certainly help if you created this image in Photoshop and it had an alpha channel that made everything but your watermark transparent...otherwise, whatever fill color you've got will impart a slight tint to your original clip even when you set the transparency very high.
If you've got a sequence of stills, and you've got photoshop (may be the same for Paint Shop Pro, maybe someone else could answer this), load up the first frame into Photoshop, go through the steps to apply the watermark image as a layer with transparency until you're satisfied with the result. Write down all the values that are pertinent, e.g. any layer transparencies you set, any bright and contrast changes you make, ...everything. Revert the primary image, create an "Action" and begin recording. Go through all these steps again and stop recording your actions when you've finished processing your example file. Create a destination directory for all the processed images on your hard drive. Create a Photoshop Batch command and select the folder with the images to be watermarked; then point the batch to your newly created output folder so it'll save the processed files there. Run the batch and, as quickly as your computer can handle it, you'll have a watermarked sequence of images ready to be assembled into a video clip. Specifics for Actions and Batch processing can be found in the Photoshop help files...they've gotten MUCH better and more complete over the years!
In the interest of brevity, I'm not going to detail every step of the process. Using the description above, you should be more than capable of using each program's help files to get the specifics (and maybe a search on the internet or two). I'm not trying to be an a** about this, but I've found a little reading in the help files actually helps more. When I was learning this stuff myself, I was always more interested in the "how" of things rather than the DETAILS of the "how."
OK, now I've screwed up the brevity thing anyway, but I hope this helps. If you've got some other program that you're using, reply to this post and I'll see if anything is different for that app. The steps above are general enough that they're pretty program-independent. Good Luck.