As of Ghost 2.26.0 you no longer need to use Markdown or HTML cards [for captioning images], the image card gives you the option of setting both the caption and alt tag.
However — and please go gentle as a newb in these here parts 
— if the image is mid-text and all the text is in markdown for the footnote feature, then there is no way to use the image card. In other words, if the text is split so that the post entails markdown, image card, markdown, the footnotes are not displayed as one would hope in the footer. Rather, the result is text, footnotes, image, text, footnotes.