I ran your code and found the image did not appear when the anchor
dimensions were too small. (I also used SpreadSheetReadBinary
instead of writing the spreadsheet to disk, but that should not make any difference). Try this example from the documentation. If the image is visible, that suggests a problem with the dimensions. To fix it you could either increase the anchor dimensions or use image functions to reduce the size of your image.
<cfchart format="png" name="image_var">
<cfchartseries type="line">
<cfchartdata item="Point1" value="-50">
<cfchartdata item="Point2" value="-25">
<cfchartdata item="Point3" value="1">
</cfchartseries>
</cfchart>
<cfset sObj = SpreadsheetNew()>
<cfset SpreadsheetAddRow(sObj, "")>
<cfset SpreadsheetAddImage(sObj,image_var,"png","1,1,7,6")>
<cfheader name="Content-Disposition" value="inline; filename=testFile.xls">
<cfcontent type="application/vnd.msexcel" variable="#SpreadSheetReadBinary(sObj)#">