When you are composing a new post for a blog the toolbar across the top of the editing window has an icon that looks like a little photo or picture. When you point your mouse at the icon it will say something like "add image". When you click on the icon an upload window opens. You can upload a photo from your computer by clicking the browse button and finding the file on your computer or you can upload a picture from a website by putting in the URL where the photo is located.
You can find detailed instructions by clicking on the help in the blog's toolbar and clicking on image or media.