HubSpot does not currently provide a thumbnail/image caption for images inserted via HubSpot's WYSIWYG (what you see is what you get) editing interface. Alternatively, after the image has been uploaded, you can clone it and take advantage of the image editor to add text onto the image. Click here for further details on image editor options.
Or, if you wish to see text immediately beneath your images in a caption-style layout it is necessary to use the WYSIWYG's table feature. From the content editor in the module where the image should appear click Table > Insert table > create a 1 x 2 table.
When the table has been created you may insert the image into the top row of the table and insert the "caption" in the row beneath, as seen below:
Note: The lines you see that form the table around the image and caption will not appear on a live page.