This question has been asked many times, I request you to check the "search for a question" option and without asking for such repetitive questions. you can get the answer.
I am trying to give the answer once again.
Just go through the following steps:-
Step(1)-Just open your steemit account and click on post a blog/create a post.
Step(2)-Then look for the link of uplaoding the image in your "create a post" and you can see that as shown in the image below. Just click on "selecting them" and choose your image you wish to upload.
Step(3)-After uploading the image url will appear and from that url, select the highlighted portion of the link as shown in the image below.
Step(4)-Then copy and paste that highlighted portion of the link in the "image add link" option in musing as shown in the image below and press the "add" button and you are good to go.
Please note that there other methods also, but I generally follow this way to upload images in musing and that is why described that in details.
Thank you and Have a great day.