ResizePixel - Free image editor to crop, resize, flip, rotate and convert images online
Vectorizer.io - Fast and easy way to vectorize an image online
Bulk Resize Photos - Resize large photos in bulk
Autotracer.org - Free online image vectorizer
PQINA Photo Editor - A free, fast, mobile-friendly, online photo editor to quickly edit pictures.
potrace - Open source program that transforms bitmaps into vector graphics.