If you are looking for image annotation tools you can find the unlimited number of online services and free tools available to annotate pictures into different formats. However, if you are seeking for tools useful for medical imaging annotation in order to build ML data sets it would be difficult to find the suitable tools or any other software that can annotate pictures for you and such images is going to be used for other purpose.
Actually, when you need such annotate images for machine learning you need to keep the annotation quality at highest level. And image annotation done with free tools or online software would be compromised with the accuracy. And here you have mentioned that person going to use these annotated pictures for machine learning can interpret the images but not skilled or expert enough to use codes and algorithms to develop the machine learning datasets or cannot utilize such annotated images for machine learning.