How would I go about acheiving an effect similar to that on this site's portfolio page Solid Giant, with CSS and HTML?

I had thought that just putting something like this would work: 

a img{
    margin-top: 5px;
}

a img:hover{
    margin-top: 0px;
}

But it did not work, even if I put the :hover on the link instead of the img. I scoured his code and css but I could not for the life of me figure this out. Help please :)

css hover onhover
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