Search engines can’t decipher the text in an image (at least not yet). Whenever you upload an image to the website, there are two additional fields called the “Alt Text” and the “Title”.
The “Alt Text” was initially created for visually impaired (blind) people so that their computer could tell them what the image was about. For example, a photo of Mt Rainier may have the Alt Text of “Mt Rainier in the summer”. Search engines then realized this was a great way to know what an image was about, and thus this is used to help is appear in searches for “mt rainier”. The rule of thumb here is to remember that this is meant to help blind readers understand an image, so focus on making it useful rather than loaded with keywords.
The “Title” text is an extra descriptor, designed to provide additional information for someone who CAN see the image. A great title text may be “Mt. Rainier – Photo by Rowland Studios May 2014 from the top of Crystal Mountain Gondola”. This helps identify which mountain they are looking at and tells them a little more detail about the photo. This has very little impact on SEO, so just focus on great descriptions.
You don’t have to worry about optimizing photos that are posted to Facebook as these aren’t indexed by Google, just worry about the ones you post up to the website.