OK, it has been a "few weeks" and JMP 15.0 is finally out.
This means you can take advantage of new features such as graphlets that were designed to support customizations such as this.
Just RMB on the graph background, launch the Hover Label Editor and add the following script (based on @txnelson solution, thx!) in the Graphlet Picture tab:
dt = local:_dataTable;
pic = dt:picture[local:_firstRow];
pic2Idx = If(local:_firstRow == NRows(), 1, local:_firstRow + 1);
pic2 = dt:picture[pic2Idx];
hlb = H List Box( pp = Picture Box( pic ), Picture Box( pic2 ) );
hlb << get picture;
No need to add new columns. A graph with pinned tooltips would look like the following:
For more information on graphlets, check https://www.jmp.com/content/dam/jmp/documents/en/support/jmp15/using-jmp.pdf, page 512.