Friendica is using BBCode for formatting of the postings @Dave for the ALT text of images, you can place the text in the middle of the [img][/img]-Tags. E.g.
[img=http://example.com/img/this_is_an_example.jpg]And this would be the ALT text for the image used on mouse over etc.[/img]
Have a look at your nodes BBCode documentation for more information about the implementation of the BBCode in Friendica, and things like the "abstracts" which other parts of the Fediverse use for CW.