You can use a new table to create what appears to be an image with a caption in the HTML editor.
- Open content file
- Create new table that contains 1 column and 2 rows
- Use Table Properties to size the table width to match the image width
- In the top cell of the new table, insert the image
- In the bottom cell, type in or paste in the text for the caption
- Use Cell Properties to align the text either left, right or center