How can I add text to my block that has just an image?

You can always add (or delete) elements to (from) a block. Click the green + icon (Add content) at the top right corner of the block.

Click Add next to the element(s) you want to add and Close to close the pop up.

You can change the order of the added element if necessary. Click the element that should be moved and then the Move up or Move down button.
You delete an element by clicking first at the element you want to delete and then the Delete button.