PhotoPenguin App - PhotoPenguin is a completely free batch photo editor with support for cropping, resizing, adding watermarks, and converting photos to other formats. It works on the user's device, using the capabilities of modern browsers.
Autotracer.org - Free online image vectorizer
Batch Image Resizer - Resize, crop, shrink, flip, exif-rotate, convert, enhance, process multiple pictures and photos with professional software! 120+ Actions, 30+ Image Formats
Vectorizer.io - Fast and easy way to vectorize an image online
ImBatch - ImBatch is a batch image processor with a nice graphical user interface.
potrace - Open source program that transforms bitmaps into vector graphics.