From the documentation:
...This parameter expects a valid path to a png or jpeg file. If setting this path from an attribute, the user must ensure that the image exists on disk at the time the features hit this transfomer...
So I think this wont work. You can choose custom HTML, so I think if you put something like
<img src="data:image/png;base64, iVBORw0KGgoAAAANSUhEUgAAAAUAAAAFCAYAAACNbyblAAAAHElEQVQI12P4//8/w38GIAXDIBKE0DHxgljNBAAO9TXL0Y4OHwAAAABJRU5ErkJggg==" alt="Red dot" />
it will work the way you need. Not tested but I expect this to work.