Unused graphic files are included in Impress's .sxi files:
After inspecting the inside of an .sxi file, I noticed that there are 
graphic files included in it which are not actually used in the presentation.
These are files that I used in the presentation but later on I decided
not to use.

To reproduce:

1. Create a new presentation, using all the defaults.
2. Type some text in one slide.
3. In Format->Page->Background, choose a bitmap as the background.
   For instance, pick the "leaves".
4. Say Yes to "Background settings for all pages?"
5. Save the file to "test1.sxi"

6. From a Linux command prompt, do "unzip -v test1.sxi"
   and we get a list like:

> unzip -v test1.sxi 
Archive:  test1.sxi
 Length   Method    Size  Ratio   Date   Time   CRC-32    Name
--------  ------  ------- -----   ----   ----   ------    ----
   17371  Stored    17371   0%  09-19-02 13:59  c7cbf147 
Pictures/100000000000005E0000005E513A4C71.png
    5205  Defl:N     1055  80%  09-19-02 13:59  faa40899  content.xml
   30846  Defl:N     2789  91%  09-19-02 13:59  37f1503d  styles.xml
    1153  Stored     1153   0%  09-19-02 13:59  79e2909d  meta.xml
    9949  Defl:N     1573  84%  09-19-02 13:59  9bf9f658  settings.xml
     876  Defl:N      286  67%  09-19-02 13:59  f501bd6e  META-INF/manifest.xml
--------          -------  ---                            -------
   65400            24227  63%                            6 files

   ...so at this point all is fine.  The PNG picture is the leaves picture.


7. Continue working on the same file: 
   In Format->Page->Background choose a different bitmap as the background
   image.
8. Say yes to "Background settings for all pages?"
9. Save the file to "test2.sxi"
10. From a Linux shell, do "unzip -v test2.sxi"
    and we get a list like:

> unzip -v test2.sxi
Archive:  test2.sxi
 Length   Method    Size  Ratio   Date   Time   CRC-32    Name
--------  ------  ------- -----   ----   ----   ------    ----
    8149  Stored     8149   0%  09-19-02 14:05  e321cd00 
Pictures/100000000000005E0000005EB758AAC8.png
   17371  Stored    17371   0%  09-19-02 14:05  c7cbf147 
Pictures/100000000000005E0000005E513A4C71.png
    5205  Defl:N     1055  80%  09-19-02 14:05  faa40899  content.xml
   31000  Defl:N     2806  91%  09-19-02 14:05  f8434e1e  styles.xml
    1153  Stored     1153   0%  09-19-02 14:05  efda7179  meta.xml
    9949  Defl:N     1573  84%  09-19-02 14:05  9bf9f658  settings.xml
     999  Defl:N      296  70%  09-19-02 14:05  a883fbd2  META-INF/manifest.xml
--------          -------  ---                            -------
   73826            32403  56%                            7 files

    ... now there are two PNG pictures.  One of them is the "leaves" and the
other one is the one we chose in step (7).  However, the presentation only
shows one of them.  In this case the unused picture does not take up 
much space, but the unused image could be a large file that the user has
tried and then decided not to use.

        I discovered that the file "styles.xml" inside "test2.sxi" includes
references to both PNG files.  However, the presentation that you see when
you open test2.sxi in OpenOffice.org Impress shows only one of the images. 
The other one cannot be seen.
        As far as I can tell, it is not a matter of an image being placed
underneath the other one.  All I do is choose a background from the
Format->Page->Background, and then decide I want to change it and choose a
different one.

        If I close OpenOffice.org, reopen it, and open test2.sxi again,
and then immediately save the file again,  the file test2.sxi still
has the unused graphic files inside.