how to make block on cover pic bigger

I added a text block onto the cover picture and as the picture has a lot of colors I applied a white background to the text block. But I am not able to make the white background bigger. I want to have a big white background on the cover pic and there then the text. How can I achieve this?

Thank you in advance and BR

Angelika

PS: now really my last question for today. ??

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