Alternative Text is a textual description of a graphical object, usually an image. It can be used in web browsers in place of displaying an image. People with visual disabilities need assistance in understanding what message an image is trying to convey because they cannot see, or have difficulty seeing the image. In addition, some web browsers do not support images. For people with visual disabilities the Alternative Text is read by screen readers to describe an image.
The purpose of Alternative Text is to allow a person who does not see the image to be able to extract the same meaning from the image as someone who can see it.
Sometimes alternative text can be very brief - less than 5 words. Sometimes it needs to be much longer. The way the image is used in the presentation will determine how much textual description it requires. For example, if you have a presentation on memories from your home town and you include a picture of your elementary school, your alternative text might be as simple as "Third Street Elementary School." However, if your presentation is on the architectural history of your home town and you include the same image of your elementary school, your textual description will probably need to be much longer in order to describe what architectural features are being highlighted in the image.
If your textual description is less than 10 words, typically you would use an alt attribute. If it is longer typically you would use a long description. It is possible to use both the alt attribute and the long description. In this case someone using an assistive technology would read the alt attribute and then have the option of reading the long description.
The first and most important thing to ask is, "What message is this image conveying?" Based on that answer then choose whether to use a brief alt attribute or a more in depth long description.