Html.RenderAction
has to be called within a script block and can't be called in mark-up.
As an alternative in markup you would use:
@Html.Action("PostImagesForPost", "BlogPost", new { id = Model.ID })
For the differences on Action
and RenderAction
see here:
http://haacked.com/archive/2009/11/18/aspnetmvc2-render-action.aspx/