If you are using a full width “section background image", for best results the image should be 1920 px in width, and whatever height you prefer. This will ensure your image looks crisp on all devices (provided you begin with a good quality image of course).
Here are the steps to create a background image and text overlay.
Add a text box with the text you wish to display eg. WELCOME
Click into the Section Settings >
Click 'Background' tab > Click Image button > Choose the image from your Gallery > tick 'Full width background' > Save
The page displays as follows ...