images = ExampleData[{"TestImage", #}] & /@ {"Lena", "Elaine", "Mandrill"};

BarChart[{1, 2, 3}, ChartLabels ->
 Placed[Thumbnail[#, Tiny] & /@ images, Axis, Panel[#, FrameMargins -> 0] &]]

BarChart[{1, 2, 3},
 ChartLabels -> Placed[images, Axis, Framed[Magnify[#, .3], FrameStyle -> None] &]]

BarChart[{1, 2, 3}, ChartLabels -> (Framed[Magnify[#, .3], FrameStyle->None] & /@ images)]
BarChart[{1, 2, 3}, ChartLabels -> Placed[Pane[Magnify[#, .1]] & /@ images, Axis]]
BarChart[{1, 2, 3}, ChartLabels->Placed[Framed[Magnify[#, .3],
  FrameStyle -> None]& /@ images, Axis]]

images = ExampleData[{"TestImage", #}] & /@ {"Lena", "Mandrill"};

BarChart[{{1, 2, 3}, {4, 5, 6}}, ChartElements -> {images, None}]