From The Document Foundation Wiki
- Have paragraph marks turned on (helps you to see what's happening, ▸ ).
- Press ↵ Enter at the end of a paragraph to create a new blank paragraph.
- Change the new paragraph’s style to Figure. The paragraph marker should now be centered on the page.
- Choose ▸ or use copy-and-paste to insert the image.
- Adjust the size of the image. Hint: Select one of the corner handles and drag to the desired size. The resizing will keep the image proportions.
- With the image selected, right-click and choose Caption. Select Figure as the Category (if it shows anything else in that box) and type the caption. If Figure is not one of the choices, type the word Figure in the Category box to create that category.
- When adding the caption, LibreOffice automatically adds a frame that contains the image and the caption paragraph. That frame needs some tweaks.
- Select the frame, right-click on it, and choose ▸ .
- Select the image inside the frame, right-click on it, and choose ▸ .
- Delete any spurious line break that may appear between the image and the caption.
- Click anywhere outside the frame to deselect it, then click in the caption line and change the paragraph style to Caption.