ThumbsUp

ThumbsUp lets you create thumbnails of one or more images just by dragging and dropping files over its icon on the Dock. It doesn't only work with images, but also with videos supported by QuickTime.

The full control panel in ThumbsUp lets you specify the prefix or suffix to be added to the file name, the image's output format (JPEG, GIF, TIFF, BMP, PNG), the size in pixels or the resize percentage, the level of compression, and other properties like antialiasing or sharpening.

ThumbsUp can generate just thumbnails, thumbnails with the original image as an icon, or just the icons so you can use them with the original pictures.