This is an essay.
It contains the advice or opinions of one or more Wikipedia contributors. This page is not an encyclopedia article, nor is it one of Wikipedia's policies or guidelines, as it has not been thoroughly vetted by the community. Some essays represent widespread norms; others only represent minority viewpoints.
Each image on Wikipedia is unnecessary, as Wikipedia is a text encyclopedia. However, an image may quickly convey information (a picture of a violin compared to paragraphs describing its shape, for example), assist readers in understanding article text, or may simply show readers the subject for a sense of familiarity (most images of people in articles about individuals, for example). Many editors have a goal of at least one image per article. How then does one decide if a specific image should be placed in a specific article?
An image should not be used in an article where its use would be tenuous (an image of clouds sans horizon next to a mention of evaporation in an article the Great Lakes, for example).
An image should not be used in an article where it would crowd the text or, in other words, there are already too many images. There should be a reasonable proportion of text to image.