Try using the UserPicture method instead: seems to work (Excel 2003, not tested in other versions).
From Excel VBA Help
UserPicture Method
See AlsoApplies ToExampleSpecificsUserPicture method as it applies to the FillFormat object.
Fills the specified shape with an image.
expression.UserPicture(PictureFile)
expression Required. An expression that returns one of the above objects.
PictureFile Required String. The name of the picture file.
UserPicture method as it applies to the ChartFillFormat object.
Fills the specified shape with an image.
expression.UserPicture(PictureFile, PictureFormat, PictureStackUnit, PicturePlacement)
expression Required. An expression that returns one of the above objects.
PictureFile Optional Variant.
PictureFormat Required XlChartPictureType.
XlChartPictureType can be one of these XlChartPictureType constants.
xlStack
xlStackScale
xlStretch
XlChartPictureType can be one of these XlChartPictureType constants.
xlStack
xlStackScale
xlStretch
PictureStackUnit Required Long. The picture stack or scale unit (depends on the PictureFormat argument).
PicturePlacement Required XlChartPicturePlacement.
XlChartPicturePlacement can be one of these XlChartPicturePlacement constants.
xlAllFaces
xlEnd
xlEndSides
xlFront
xlFrontEnd
xlFrontSides
xlSides
Example
This example sets the fill format for chart two.
Charts(2).ChartArea.Fill.UserPicture "brick.gif"