In order to add images to your Musing post click on the rightmost small icon shown in the screenshot below and paste the URL of your image.
To get an image on your personal computer (that doesn't yet have a URL) you can navigate to the "Create a Post" section on Steemit.com and upload the image by clicking on the text **"Insert images by dragging & dropping, pasting from the clipboard, or by selecting them."** Then highlight ONLY THE URL as shown below, and paste it into Musing.
Hopefully in the future there is inline support for this sort of thing, but for now this is a process that definitely works. If you have an image that is already on a website, you can do it much quicker and link it directly.