From The Document Foundation Wiki
When creating labels:
- Use a line weight and font size and weight that will be compatible with the surrounding text when the image is at its final size in the document (images may be reduced in size when in the document).
- Use a white background for the area surrounding an image, when labels are outside the image. You can use a light color for the background of labels placed inside an image (to help them be noticeable), but choose a color that will print as pale grey in black-and-white.
- Ensure all numbers or words in the labels are lined up neatly.
- When labelling dialogs, it is often best to put the labels above or below the image, not to the sides, to help fit the image on the Writer page. With small images, labels beside the image may work well or better; this is a matter of judgment.