I often encounter this question in here. Unfortunately at the moment, the bold/italize and adding images can only be access through a desktop or through the web version of Musing. Refer to the pic below. Or by using Keyboard shortcuts:
If you are on mobile, the easiest way to do this is to enable the "Desktop-site" view on your phones internet browser (i.e Google Chrome). This setting is usually found under "Settings".
While yeah it is kind of confusing when a new users enters in Musing.io and looks out for those features to make the comment a little more informative and beautiful. I had that same problem but got to learn it from the users in here.
To add image click on the sign directed by arrow
Next upload the image in Steemit.com (do not post it) and from there copy the link of the image and paste it in the "Paste in the image url" and than click add. That is it for uploading image.
While as for the making text bold or Italic
Select the line or word and click the "B" or "I" than the phrase or word will turn to Bold or Italic or if you select the both than you will get the phrase BOLD and Italic.