Click on the Media section, click on the folder you want to upload it to, and click the Click to Upload link. You can also drag and drop the file onto the browser window. To swap out an image, the new image must be named the same as the old one. Navigate to the image in Media and click Browse, then save.
To embed, click where you want the image. Select the picture icon.
Browse to the folder and select the image. Folders appear as grey boxes. Click on the name of a folder to go into the folder, click Select if you want to choose the folder itself. You can resize it later by dragging corners.
If you need to wrap the text around the image, click on the image and go to Formats, selecting "Picture Left" or "Picture Right".
You can align using the alignment tools.
If you need more control over the margin, click the HTML button and look for an <img src="URL" />. Add style="margin-left:10px;" after the src="URL" and before the />. Change the margin-left to margin-right, etc.