Gatsby Image CSS Position:Absolute
Gatsby image adds inline css of position:relative and overflow:hidden on rendering. This ends up oversiding any CSS that you try to add with a className. To conteract this you will need to add inline css on the element itself.
ex:
<Img
fluid={data.node.childImageSharp.fluid}
alt={data.node.frontmatter.ImgAlt}
className="absoluteImage"
style={{ position: absolute }}
/>