Thanks for the ideas! I used the above ideas to make a macro to do a bulk file conversion--convert every file of one format in a folder to another format.
This code requires a sheet with cells named "FilePath" (which must end in a "\"), "StartExt" (original file extension), and "EndExt" (desired file extension). Warning: it doesn't ask for confirmation before replacing existing files with the same name and extension.
Private Sub CommandButton1_Click()
Dim path As String
Dim pathExt As String
Dim file As String
Dim oldExt As String
Dim newExt As String
Dim newFile As String
Dim shp As Picture
Dim chrt As ChartObject
Dim chrtArea As Chart
Application.ScreenUpdating = False
Application.DisplayAlerts = False
'Get settings entered by user
path = Range("FilePath")
oldExt = Range("StartExt")
pathExt = path & "*." & oldExt
newExt = Range("EndExt")
file = Dir(pathExt)
Do While Not file = "" 'cycle through all images in folder of selected format
Set shp = ActiveSheet.Pictures.Insert(path & file) 'Import image
newFile = Replace(file, "." & oldExt, "." & newExt) 'Determine new file name
Set chrt = ActiveSheet.ChartObjects.Add(0, 0, shp.Width, shp.Height) 'Create blank chart for embedding image
Set chrtArea = chrt.Chart
shp.CopyPicture 'Copy image to clipboard
With chrtArea 'Paste image to chart, then export
.ChartArea.Select
.Paste
.Export (path & newFile)
End With
chrt.Delete 'Delete chart
shp.Delete 'Delete imported image
file = Dir 'Advance to next file
Loop
Application.ScreenUpdating = True
Application.DisplayAlerts = True
End Sub