You can create a C++ class that inherits from RichTextBlockImageDecorator and add the proper UCLASS properties so it can be configured in blueprint (i.e. Blueprintable, BlueprintType)
The signs for referencing a row in the Text Style Set data table in your example should also be reversed: “(LessThanSign)rowname(GreaterThanSign)your text(LessThanSign)/(GreaterThanSign)”