Alt text, as known as Alt Attributes is used for describing the content of an image. Since search engine crawlers cannot see what is in an image, they rely on the alt text to tell them what an image is about.
In Portfoliobox, the image title serves as the alt text. It is important that the image title is as descriptive and concise as possible. Similar to site indexation, it takes some time and is up to the search engines whether they index your images or not.
Here is an example of how you should write your alt text (image title):
Too general: coffee
OK: black coffee
Good: black coffee in a white ceramic mug