Hello @MichalKoren ,
At the minute, the legend can only be placed in the lower left hand corner of the map. If you would like to immediately customize that legend location further, I would recommend embedding a web application like ArcGIS Web Appbuilder to choose the location of that legend. I'd also suggest posting on our ArcGIS Ideas page to suggest adding a custom legend location option to ArcGIS StoryMaps in the future.
Thank you,
Abby