This tool adds a solid border around your photos. Set the width, pick any colour (white, black, cream and other presets, or your own), and round the corners if you want a softer look. The polaroid option adds a deeper bottom margin, like an instant photo, and can write a short caption in it.
Widths can be set in pixels or as a percentage of each photo's shorter side. Percent is the default because it gives a batch of different-sized photos the same look: a 4% border looks the same on a small screenshot as on a 12-megapixel photo. You can frame up to 30 images in one run and download them one by one or as a ZIP. The photo itself is not cropped; the border is added around it, so the image gets larger by the border's size. Photos up to about 16.7 megapixels keep their full size; bigger ones (such as 24 or 48 MP phone shots) are scaled down to that, the most a browser canvas can safely hold.
Rounded corners need transparency, so on Auto the tool saves them as PNG; if you choose JPG, the corners are filled with white. The caption uses one plain font and the same text for every image in the batch. Everything happens in your browser, so your photos are never uploaded.