Hello @mielele,
The option to add the custom data from the drop down option Custom is only in the Sections > Counter . The other sections that are available in the theme can be set using page /post / Category .
Also you can add your desired picture through Featured image section of page/post edit screen .
For more clear info refer to below link :
https://image.prntscr.com/image/GGowV5ScSRGMsRud7T1bcQ.png
Hope this helps.
If you have any confusion further please let us know .
Best Regards!!