How can I outline an image with a transparent background?

For a quick, rough and ready result, for an image that has a single colour,

open Keynote and add the image to a slide:

go to Format > Image > Instant Alpha

The pointer will change to a cross hair with a target spot

click and drag very slowly over the part of the image you want to remove

when selected, press the delete key to remove the background

go to, File > Export and save as PNG file

For images that have many colours in the background area, use Preview application located in the Applications folder open the image in Preview and click the markup button in the toolbar, this looks like a pen nib, located next to the search bar

click on the selection tool (first button on the left of the toolbar)

and choose the lasso tool

draw around the area you want to remove

invert the selection (Edit > Invert Selection)

File > Export and save as PNG file

These tools will take some time to practice to use !

It seems to only work if you have used Keynote to set the transparency: Fortmat > Image > Instant Alpha

(This might also have something to do with the specs of the alpha channel)

You are using the correct tool in Preview and you were able to draw a line.

You say in the video "It doesn't follow my outline"

That's because you are not drawing accurately enough. You use the draw tool just as you would using pen and paper.

This took 10 seconds to draw a quick outline, so take your time.

I wasn't suggesting you should be able to draw this in 10 seconds, rather that's the result I achieved in 10 seconds, so taking time and care its certainly possible using Preview.

I do not know of a simple one step automatic way to draw an outline as you want.

There is an outline tool in photoshop, which I have used, however the background must be manually painted out very accurately first, this is much more tedious and time consuming that the method I described using preview.

I feel for you, hendry. The replies you got here from Gary obviously wasn't at all what you were looking for. But there definately is a solution, and it's super simple:

In Keynote just select your image with a transparent background and then use Format > Style > Border > Line.

(I also use Shadow for work like this.)

Hint:

If you start out with an image that already has transparency then it’s enough to “sacrifice” one single pixel of it by applying the Instant Alpha tool. That’s what I did with the right hand cat picture above.

Use the drawing tool in the tool bar and select the colour from the shape border colour picker:

(It also works on other images than cats)