To have AutoViewer Pro display the middle image first, you will need to edit the 'StageManager.as' source file and then publish a new SWF file using Adobe Flash: http://simpleviewer.net/autoviewer/pro/support/#repub
On line 231 of the 'StageManager.as' file, change:

index=0;

... to:

index=(Math.round(mImageCount/2))-1;

Try this:
Change line 231 of 'StageManager.as' back to:

index=0;

Now change line 163 of 'StageManager.as' from:

showImage();

... to:

showImage((Math.round(mImageCount/2))-1);

Did you change line 231 back to 'index=0;' before changing line 163?
Also, try clearing your browser's cache before reloading the gallery to see the changes.

I tested my most recent suggestion myself and it seems to work well.
Line 163 is the initial call to display an image in the gallery.
Changing this line to 'showImage((Math.round(mImageCount/2))-1);' initially displays an image near the middle of the gallery.
The next image displayed after the last image (if clicking 'Next' or if the slideshow is automated) is image #1 (not the middle image).

Try making the modification (line 163 only) with a fresh copy of 'StageManager.as' to ensure that no other modifications have been made.
Also, make sure you clear your browser's cache in order to be sure that your browser uses the newly published SWF file.