Autotracer.org - Free online image vectorizer

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.

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.

Batch Image Resizer - Resize, crop, shrink, flip, exif-rotate, convert, enhance, process multiple pictures and photos with professional software! 120+ Actions, 30+ Image Formats