ImBatch is a great free app that lets you modify multiple images, all in one go.
This batch processing function is a real godsend, allowing you to resize, rotate, add shadow, flip, round corners and convert colors in multiple images in one single batch. ImBatch has various options, and not all of them have helpful alt text, so it might take a few goes to get to grips with it. Basically, add the images on the left (drag and drop or use the little add button), and specify the modifications on the right.
One of ImBatch's best features is the ordered way you can apply the changes. By using the Add Task button, you can specify the changes and the order in which you want them applied. As you add tasks, you can make further specifications, such as what size you want the images to be saved as.
Finally, pick the save specifications (name, format, location). Then hit ImBatch's large green go button (top left) and off it goes!
ImBatch is an all-round great app. It's free, highly configurable, fast and works as expected. Download it now!