Alt attributes (also known as alt tags or alt text) are short descriptions of images, mostly used by screen readers for vision impaired visitors.
As such we want to succinctly describe the image, and integrate keywords naturally. Do not stuff or force keywords.
Example, if it’s an image of a propane freezer that is black in color, the alt attribute might be “black propane freezer,” if we have a model number we might want to include it also. The content writer will probably not know what image will be used, so content writer can just add the main targeted keywords and the developer will compose Alt tags
Rule of thumb is to keep alt attributes to around 3 to 6 words max.
If the page is targeting a specific location it’s recommended to add the targeted geo in the alt text.