I have opted for

"profileImage" : {
        "phid" : "PHID-FILE-w5kjphncwdhjgyazny6j"
      },

instead, as that will allow

"profileImage" : {
        "phid" : "PHID-FILE-w5kjphncwdhjgyazny6j",
        "uri": "some/uri",
        "50x50": "some/other/uri"
      },

etc in the future instead of

"profileImagePHID" : "PHID-FILE-w5kjphncwdhjgyazny6j",
"profileImageURI": "some/uri",
"profileImage50x50URI": "some/other/uri"

Let's just do profileImagePHID instead of the sub-array, this is more consistent with lotsOfOtherStuffPHID in modern calls.

Specifically, it's unlikely (but possible) that we'll add profileImageURI here, but profileImagePHID makes it clear that the value isn't a URI, like authorPHID makes it clear that the value isn't a username and activeDiffPHID makes it clear that the value isn't a vanilla ID, in a call like differential.query. This is super nitpicky overall but annotating PHID fields with ...PHID just improves consistency and clarity a little bit.