Custom image annotation are also not working for me.
Setting the markerStyle to 'img':
{
		point,
		color: config.strokeColor,
		markerStyle: 'img',
		imgIdx: 0,
},
...and even with rendering the image array with a valid PNG:
[
		{
				url: config.logo.url,
				height: config.logo.height,
				width: config.logo.width,
		},
])
...it just ends up defaulting back to the balloon.