Yes, you can.
You can insert images (or any other files) using HTML while creating custom tasks, but you would need to host them somewhere (like imgur).
You need to get to Source code (<> icon) and paste the image using <img src=“link_here”></img>. In case you want to attach a file, you need to do that using <a href="link_here">link text here</a>.
In custom multiple-choice tasks you can simply copy and paste the image. If you need to insert link into text, then that is not possible in the editor, but it can be done by copying the linked text from a different editor.