How can I use Picasso to add icon to Marker?

Picasso provides a generic Target interface you can use to implement your own image destination. Specifically, you will want to override onBitmapLoaded to populate your marker.

A basic implementation is given below.

public class PicassoMarker implements Target {
    Marker mMarker;

    PicassoMarker(Marker marker) {
        mMarker = marker;
    }

    @Override
    public int hashCode() {
        return mMarker.hashCode();
    }

    @Override
    public boolean equals(Object o) {
        if(o instanceof PicassoMarker) {
            Marker marker = ((PicassoMarker) o).mMarker;
            return mMarker.equals(marker);
        } else {
            return false;
        }
    }

    @Override
    public void onBitmapLoaded(Bitmap bitmap, Picasso.LoadedFrom from) {
        mMarker.setIcon(BitmapDescriptorFactory.fromBitmap(bitmap));
    }

    @Override
    public void onBitmapFailed(Drawable errorDrawable) {
    }

    @Override
    public void onPrepareLoad(Drawable placeHolderDrawable) {

    }
}

You would use it like this --

marker = new PicassoMarker(myMarker);
Picasso.with(MainActivity.this).load(URL).into(marker);

Note Picasso only holds a week reference to the Target passed to into. Therefore, the marker reference needs to exist until the image is loaded to avoid have the callbacks being cleaned up by the garbage collector.