Fast, intuitive, simple, free. What else could you ask for? Non-Mac app store perhaps but the creator Olga Kacher offers Resize Master free to attract photographers to her other outstanding paid apps like Panorama Stitcher and Focus Stacker which get rave reviews on the app store.
Other programs allow you to add text watermarks. Resize Master assumes you are a pro and have a transparent PNG logo ready to go (and in different sizes depending on the job - no logo proportional resizing built-in. Add the right size before watermarking/processing. Not everyone needs a watermark so many photographers won't even feel the slight clunkiness here and it keeps the interface dead simple.