Graphic elements can have two slightly different meanings, and while I see this is posted under "Literature," I am not sure which context is being asked about. I will address both. As a general matter, something that is graphic is something that is visual, and that definition is the underpinning for both contexts I will discuss.
In literature, we call graphic elements those that have a vivid enough description to allow the reader to see what is being described. Sometimes I hear this term applied to a passage that is quite violent or sexual in nature, and you will hear parents complain from time to time that they do not want their children exposed to graphic elements. Nevertheless, this term is applicable to any description that allows us to easily visualize. This could be a description of a person, natural phenomenon, building, or action. One book that immediately comes to my mind when I hear this term is Young Men and Fire by Norman Maclean. He goes deep into the heart of a fire, and when you are done reading, the images you are left with are vivid and seared into your memory.
In poetry, graphic elements have a bit of a more specialized meaning. There is a strong visual aspect to poetry: the layout of the words on the page (haikus are a good example), or the use or absence of capitalization, as seen in e.e. cummings's poetry. Poets takes care with the graphic elements, which can contribute to the understanding and enjoyment of the poem. Otherwise, the poet could just write in paragraphs. Lines are broken up into the units they are can be just as much due to graphics as they can be done for rhythm or rhyme. Found poetry, certainly, is dependent upon its graphic elements.
No matter whether you are discussing literature in general or poetry in particular, remember that either way, graphic elements are visual elements—either those you can visualize because of the narrative or those you can appreciate because of their particular layout on the page.