All it takes is simple HTML coding to add captions to externally hosted images. First, have the URL of your image on hand. Then, write an open figure tag followed by a close figure tag using the Text Editor. Next you add the image HTML tags along with the attributes of src, alt, height, and width. Add the image URL in the src attribute and fill in the others accordingly. Next, you add the caption tag inside the figure tags either before or after the img tag. Finally, you add the caption text in between the figcaption tags.
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How To Add Captions To Externally Hosted Images