Add captions to board items to add context.
Hover over the image and click the 'Aa' icon at the top right corner (The caption text box will pop up below it).
Type your caption into the box (it will save automatically when you click away from the caption text box).
Hover over the caption text to choose to either left/right align or centre the text below the image. You can also choose to have your caption above or below the image by altering the 'position'.
Captions are different to text blocks in that they are attached to the image, so you can move board content around without losing the text that goes with it.