Add Outline to Image
Add an outline to an image: a stroke of any color and width that follows a PNG cutout's shape, or the photo's frame. Saves as a transparent PNG, free.
What is an image stroke?
An image stroke, or outline, is a band of solid color drawn around the edge of a picture. On a cutout (a PNG with its background removed) it follows the subject's shape, the way a die-cut sticker keeps a white margin around its print.
Design apps can put a stroke inside the edge, across it or outside it. This tool always draws it outside, so it doesn't cover any of the picture.
How to add an outline to a PNG
Load a PNG with a transparent background and the stroke will follow the subject's shape. A picture without transparency gets a border around its rectangle instead. The page opens on the Cut out preset, with No backdrop ticked and Shadow at 0, so you only see the picture and its stroke.
Border sets the width as a share of the frame's height, up to 0.08, and Border colour sets its color, which is white by default. Size controls how much of the frame the picture fills. If you want a flat Backdrop color, untick No backdrop, and raise Shadow for a soft shadow just under the shape. The Sticker, Thick border, On pink and Dark border presets turn both back on.
Settings for a WhatsApp sticker, a printed die-cut sticker and a white product on a white page
WhatsApp wants stickers at exactly 512 × 512 px in WebP, 100 KB at most, and recommends an 8 px white stroke. Start from a square transparent PNG 512 px wide and keep Cut out. Set Border to 0.016, which is 8 px of a 512 px frame, and Size to 0.9, which leaves about 25 px of clear margin on each side for the stroke. Save a PNG, then turn it into WebP with Image Compressor; its readout tells you when you're under 100 KB.
For a die-cut sticker from a print shop, the blade follows the outer edge of the border, so a wider border forgives a cut that lands a little off. On a picture 2000 px tall printed at 300 ppi, Border 0.02 comes to 40 px, about 3.4 mm, and Thick border's 0.05 is about 8.5 mm. Leave Shadow at 0. It falls outside the border, right where the blade would cut it off.
For a white product on a white web page, keep No backdrop ticked and set Border colour to a near-black like #16161a, the color the Dark border preset uses. Then put Border somewhere between 0.004 and 0.008. That thin, the stroke looks like a drawn edge instead of a sticker margin.
How is the outline traced?
The tool reads the picture's transparency. Any pixel that's at least half opaque counts as picture, and the rest is empty space. Then it measures how far each empty pixel is from the nearest bit of picture. That's a distance transform, done in one pass along each row and one down each column.
Every pixel within the Border distance takes the Border colour. You get a stroke of one even width all the way round, with rounded outer corners: what image processing calls a dilation with a round brush.
Why does the outline follow the rectangle instead of the subject?
Because the tool only reads transparency. A JPG, or a PNG that still has its background, is solid right out to its edges, so its outline is the frame. Take the background out first. Green Screen Remover keys out a plain backdrop and saves a transparent PNG you can bring back here.
If the stroke stops in a straight line at the edge of the frame, the subject still reaches that edge even after Size has shrunk it. The stroke needs clear space as wide as itself on every side. So keep Size at no more than 1 minus twice the Border, which means 0.9 for a Border of 0.05.
Saving an outlined cutout
PNG keeps the space around the outline clear and the stroke's edge exact. WebP keeps the transparency too, in a smaller file. Size leaves a clear margin around the sticker, and Trim Transparent Edges cuts that away before you drop it into a layout.
Video has no transparency to follow, so each frame just gets a border around its rectangle. If you want the sticker shown on a laptop or a mug, the Mockup Generator hub has die-cut and holographic looks.