It counts as 1 image.

But SS does count vectors and JPGs separately, so if you upload a vector image and then upload a raster version (JPG) of that same vector, essentially that one image counts as 2. Not every vector gets uploaded as JPG, and some vectors are rejected while a JPG version of that same image gets accepted, so I wouldn't say that vectors double themselves in the total count. But they do account for a good number of images in the collection that are really just alternate versions of the same image.