The best tool to undertake the task at hand is the simpliest one that handles it
This freeware program helps one into easily batch convert images (bmp, gif, jfif, jif, jpe, jpeg, jpg, tif, or tiff) with a multitude of options, like: rotating them, mirroring them (on the X and the Y axis), changing their colors' scale (to either gray-scale or sepia), resizing them (with or without stretching), and, when the images need a frame (in order that they reach a specified dimension), adding one to them that can be a solid black one or one composed of replicas (mirrored or not, with or without the same color's scale) of the corresponding original image.

In addition, this program, as its name implies, can keep state, so that a batch job can be paused and, then, continued, at a later time.
Windows XP, Windows Vista, Windows 7, Windows 8, Windows 8.1, and Windows 10
Portable installation:
Uncompress the zip file to the folder from where you want to run the program (assuming you have both write and execute permissions on that folder) and run it from there.