Alternative text is a short description of an image thats stored in the images metadata. This description is read out when a screen reader navigates to an image. There are a few key rules to writing good alt text.
The guideline for alt text is to use 180 characters, including spaces, or less. This is because each screen reader will stop reading out alt text after an amount of characters
Screen readers will announce that the user has navigated to an image before reading the alt text. Adding in the words "Image of" or "Picture of" is therefore redundant. A mild exception to this would be specifying if the image is a photograph or a drawing when its relavent to the context of the image.
Alt text is not a space to describe an image perfectly, but rather to describe your image in how it functions in your document. If you have a picture of a cat in your pattern,
you'll have to think about why is it there. Is it just to be cute? Then simply write that it is a cat. Does it relate to the text of your pattern? How does it relate?
Maybe you would describe the cat as "a cat looking to eat yarn".
Essentially, do not over describe your image.
In each program theres a way to mark your image as decorative. This is for cases like a colored box behind text, or an image that doesn't add anything to the text.
When marked decorative an image will either be read out as decorative or it will be skipped entirely.
When in doubt if the image should be decorative, its better to
assume it is not decorative and give it alt text.
Long descriptions are descriptions of an image within the text of your pattern. This is usually reserved for when you cannot sufficently describe an image with alt text alone. In this case, you should specify in the alt text where someone can find your long description. It is also useful to label your long description as a long description so that screen reader users can easily find it and so sighted users know what it is.