HubSpot does not currently provide a thumbnail/image caption for images inserted via HubSpot's WYSIWYG editing interface. This feature is currently in development and may be tracked here.
With that said, after the image has been uploaded, you can clone it and take advantage of our image editor to add text on to the image. Please click here for further details on our image editor options.
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 Insert > Table > and create a single column 2 row 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. When you have finished editing your image and caption click Save to push the changes live.