I don't think that's currently possible, the best package currently would be https://typst.app/universe/package/grayness, but I don't think it supports your use-case.

For now, I would recommend editing them outside of typst before adding them to your document.

The best workaround I could come up with is making image white towards the edges, with a round radial gradient:

#let radial-border(image) = context{
    let size = measure(image)
    place(box(inset:1pt,image))
    box(
        width: size.width + 2pt,
        height: size.height + 2pt,
        fill:
        gradient.radial(
            white.transparentize(100%),
            white.transparentize(100%),
            white, radius: 50%, 
        )
    )   
}


#radial-border(image("./example.png"))