This distinction is incompatible with the common meaning of equivalency or equivalents.
Equivalence relations are commonly reflexive and symmetrical.
Any thing by itself forms an equivalence group. If there are more than the one equivalent, then there are alternatives and the situation becomes interesting. Note that equivalent information focuses on fulfilling the same function. If the image is part of a link and understanding the image is crucial to guessing the link target, an equivalent must also give users an idea of the link target. Providing equivalent information for inaccessible content is one of the primary ways authors can make their documents accessible to people with disabilities.