To add a new image into a new post, first click Posts from the left toolbar, then Add New.
Enter the text of your post into the main text box. When you would like to insert an image into the text, click the Add Media button.
This will open a new window: Insert Media. To add an image you have not previously used, click Upload Files. You can either drag an image into the drop area, or click Select Files, and chose an image from your computer.
(To add an image you have uploaded previously, click Media Library, and chose the image from the thumbnails.)
Once you’ve chosen your image, it should appear as a thumbnail, with a blue box around it. You can now enter any details that you would like to accompany the image. Under Attachment Details along the right side, fill in the image’s details, such as title, caption, and description. (This information will appear underneath the image, in a lightly shaded box.) When you are done, click the blue button: Insert into Post.
When you are done, click Publish.