digiKam allows you to batch process images and not only to change the format of the image, but it even allows you to add borders, change the colour and add image effects and filters. You can also rename, recompress, resize and use batch to convert RAW files. To add borders to a number of photos go to Batch on the menu bar and select border images. Next choose the type of border you want to add to the images. You can also change the color and width of the border as well as the bevel effect if there is any. Either type in or select the target folder where you want your bordered images to be saved. Lastly, click the Add button to select the photos or images that you want a border added to and click on the Start button. The steps are the same for the other batch options as well.