You could add images to the content of a solution article, as inline images or as downloadable attachments. 

To add an inline image, please click on the "Insert Image" option in the solution article text editor. You could add an image by drag+dropping the image to the "Drop Image" space or by choosing an image by browsing through your system. You could also add a image URL or manage and maintain a repository of images on your Freshdesk Account, from which you could choose.

Note: The maximum size limit for the inline images is 5.2 MB

You could add an attachment to your Solution Article, using the "Attach File" option at the bottom-left of the text editor. This would allow you to browse through your system and attach a file to the Solution Article.