If you want to hide default content from theme then you can follow the following steps:
Go to Dashboard => Appearance => Customize => Basic Setting => And click in the Hide default content from theme => Then save it.
Please use the following steps to fix your issue:
Go to Dashboard=> Appearance=> Widget=> Banner=> Then put the Text Widget inside the banner widget=> After that please use the image url inside the content but use the image url like this
Then save it.
We hope this will be helpful to you if not then please let us know.