I'm looking for a way to add a 3 pixel black border to every file in a folder. File type is .jpg.
My OS's are Ubuntu 14.04, Ubuntu 15.10, and Windows 7.
It's a command line utility which is very suitable for batch processing of many files without user interaction.
You can use gimp on batch mode over all files in folder and run gimp scripts for visual effects like this one for borders.
IrfanView works well for me for sets of images. Here are some how-to notes
Here is my stored procedure for border insertion - Infran Procedure to add a border
gnuThumbnailer supports Windows and Linux. It is primarily for creating thumbnails of images but has an option for adding borders to images. This option would probably only work if your images are the same size.
One online tool to add borders to images in bulk is available:
https://supertool.org/image-picture-photo-borders/
You can upload multiple files, choose border color, add up to two borders, and adjust border size. The borders adjust proportionally to image size in the batch you want to process. This latter part doesn't fit the exact request here, but is useful if you have a batch of different sized images. That is, w/ a border of 3px, a large image that is 5000x5000 will end up with a small border vs. a small image that is 25x25 will end up with a proportionately larger border. The advantage with this tool is that you set the border size relative to each image and so the border size scales well.
You can use ImageMagick for that. It run on macOS, Linux and Windows. The command to add a 3-pixel black border on all sides to all JPEGs in the current directory is:
magick mogrify -bordercolor black -border 3 *.jpg