Thank you very much for the tips. If I build a shortlist from scratch using the builder, it's not possible to add html tags.
But using your links I found that if I use the CSV template to create a feature class, it offers 5 description fields and these can be populated with html tags directly to include images, hyperlinks etc. This is called the 'as is' option as described here. Esri Story Maps - FAQ
The template and field descriptions can be downloaded with this :
http://links.esri.com/storymaps/shortlist_layer_template
And here's a tip to ensure your images fit nicely into any size of detail panel. Use the max_width property with a calculation :
<img style="max-width: calc(100% - 20px);" src="...your image url..." alt=" ">