Make sure the image is imported into EverWeb at the required height and at least 2000px wide.
Check the "Full width" box in the Metrics inspector, choose the image and drag it to the required height.
Check the box if a caption is required and enter the caption text.
Set the caption font family, size, color and the background color and its opacity.
Adjust the values for top/bottom and left/right padding and set the corner radius.
Choose to situate the caption at the top or bottom of the image and set the distance from the top or bottom edge.