It depends on where, and how you are entering the text. Are you using an ordinary text box, html code, in the photo gallery description field etc? If you can tell us exactly what you are doing we'll be glad to help you out.
In the photo gallery description field, there is space to label the image. I want the titles of my paintings to be in italics. Right now I have the titles in quotation marks because I don't see an option for italics.