Drop shadow
Add a soft drop shadow behind an image, on your device. Always outputs PNG for the transparency.
Runs on your device. The file is never uploaded.
Drop shadow traces the alpha channel of your image, offsets that silhouette 10 pixels right and down, box-blurs it twice at radius 6 and composites the picture back on top. The canvas grows on all four sides so nothing is clipped. That padding is transparent, so the output is always PNG and cannot be flattened onto a colour.
Questions
Why is my shadow a plain rectangle?
Because the shadow is built from the alpha channel of your image, and a JPG is fully opaque everywhere. With no transparency there is no shape to trace, so the shadow is the outline of the whole picture. To get a shadow around a cut-out subject, start from a PNG with real transparency, for example the output of Remove image background.
What can I set?
Shadow offset X and offset Y in pixels, both default 10 and both accept negatives, Shadow blur radius in pixels, default 6, Shadow colour as hex, default #000000, and Shadow opacity from 0 to 100, default 60. Out of range values are refused with messages such as "opacity must be a number between 0 and 100" and "blur must be a whole number of 0 or more".
Does the image get bigger?
Yes, so the shadow is not cut off. The canvas grows on all four sides by the blur radius plus the larger of the two absolute offsets, and your image sits in the middle of that padded canvas. With the defaults, that is 16 pixels on every side. The original pixels are never scaled or moved relative to each other.
Why do I always get a PNG back?
Because the padding around your image is transparent, and only PNG can carry that. The output is always encoded losslessly as PNG and named with a -shadow.png suffix, whatever you dropped in. There is no option to flatten the shadow onto a solid background; add one with Add border / frame if you need it.
How is the blur done?
Two box blur passes over the shadow alpha, one horizontal and one vertical, with the radius you set. It is a simple average, not a Gaussian, so a large radius looks flatter than a photo editor would produce. A blur of 0 gives a hard-edged silhouette offset behind the image.