I have over 500 pictures that I need to watermark with text that will be different for every image. For example, if I have a picture of a building I want to watermark the Name or Address of this building. I have the unique information saved in an excel file with the image file name. Now after searching for the past three days I have not come across a tool that can batch watermark unique text for each picture. I am primarily running on a Windows, and I am comfortable using Command Line to complete this process. I am also willing to spend some money on a tool to do this so it doesn't need to be free and/or open source.
You could do this with the ImageMagick convert command from a batch file it will allow you to apply text to a file, convert the type, rescale, etc., all in a single operation - it is also free.
Alternatively you could use python, PIL (the python image library) or Pillow (a fork of PIL) and xlrd to do the whole job automatically including reading from the excel files. These are also free tools but will take a little more learning on your part. I would strongly recommend taking a backup of all your images before trying any operations on them.
Just to add - seeing some of the comments on the original question that all the tools that I mention in the answer above are: