Problem Description:

Inserting a picture after deleting first picture causes an exception

Reproducing Steps:

1. start a Spreadsheet
2. insert first & second picture
3. delete first picture
4. insert third picture

Expected:
The third picture is inserted without errors.

Actual:
An exception is thrown.

>>org.zkoss.poi.openxml4j.exceptions.PartAlreadyExistsException: A part with the name '/xl/media/image2.png' already exists : Packages shall not contain equivalent part names and package implementers shall neither create nor recognize packages with equivalent part names. [M1.12]
>> at org.zkoss.poi.openxml4j.opc.OPCPackage.createPart(OPCPackage.java:735)
>> at org.zkoss.poi.openxml4j.opc.OPCPackage.createPart(OPCPackage.java:699)
>> at org.zkoss.poi.POIXMLDocumentPart.createRelationship(POIXMLDocumentPart.java:350)
>> at org.zkoss.poi.xssf.usermodel.XSSFWorkbook.addPicture(XSSFWorkbook.java:447)
>> at org.zkoss.zssex.model.impl.DrawingManagerImpl.addPicture(DrawingManagerImpl.java:386)
>> at org.zkoss.zss.model.sys.impl.XRangeImpl.addPicture(XRangeImpl.java:2332)
>> at org.zkoss.zss.api.impl.RangeImpl.addPicture(RangeImpl.java:735)
>> at org.zkoss.zss.api.SheetOperationUtil.addPicture(SheetOperationUtil.java:103)
>> at org.zkoss.zss.api.SheetOperationUtil.addPicture(SheetOperationUtil.java:89)
>> at org.zkoss.zss.api.SheetOperationUtil.addPicture(SheetOperationUtil.java:75)
>> at org.zkoss.zss.essential.PictureComposer.add(PictureComposer.java:47)