There are two ways to specify what should be in the img-element's alt attribute.
@Html.Picture(Model.Image, ImageTypes.Teaser, altText: "My alt text")
@* Also possible to use the url (string) as parameter, instead of an image reference *@
@Html.Picture(Url.ContentUrl(Model.Image), ImageTypes.Teaser, altText: "My alt text")
It's also possible to get the alt text from the actual image (IContent).
Add a property named "ImageAltText" to your image file model
[Display(Name = "Alt text")]
public virtual string ImageAltText { get; set; }
and add the following appsetting in web.config
<add key="IPE_AltTextFromImage" value="true" />
The alt text will be rendered when using the Picture helper
@Html.Picture(Model.Image, ImageTypes.Teaser)
Note that it is necessary to use the image reference as parameter when fetching the alt text from the image.