This article explains how to set a background image to a section and overlay text. This lets you create the look of text floating over an image.
If you want to use a full width background image in your section, 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).
Add a text box with the text you wish to display e.g. "WELCOME". You can change the font and colour of the text, centre it, and add some spacing so it appears in the middle of the text box.
Click on the cog icon to enter the Section Settings.
Click the Background tab, then select Image and choose the desired image from your Gallery. Tick Full width background and Save when finished.
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