Image Description
You can use the File->Image Description dialog box to give the image a description. The description is not visible in the image, but is saved as a comment when you export the image to a LaTeX document (see Export to Single-Paged Document). If you have the Add \pdfinfo to exported LaTeX documents option selected in the Document Settings tab of the TeX/LaTeX Configure Settings dialog box, the image description will be added to an exported LaTeX document in the /Title attribute of the \pdfinfo argument.