[BITV]: Replace aria-label's of background buttons with another name

[JuliaKirschenheuter]

This is accessibility issue.

For now background buttons have aria-label's which are not really understandable, like "Soft floral (Hannah MacLean, CC0)". The aria-label have to be like "Background with abstract ..."

Each name have to unique.

[szaimen]

So we need to add descriptinos for each picture and adjust the aria-label to show these. Also currently title as well as aria-label are not translated which needs to be adjusted.