This is an effect of print spooling. Items are not added to the print queue until they finish spooling so they will queue in the order that they finish spooling rather than the order in which they are printed. Large files (with images, etc) take longer to spool than smaller ones so it is possible for items printed later to finish spooling first if they spool more quickly. You can disable print spooling (as has been suggested) but this can make printing cumbersome since all applications will then block on print jobs until they are finished rather than letting the spool do the work.
An alternative may be to query the print queue in your own application - don't send your next print job until you get confirmation that it has finished spooling (if spooling is being used). This gets around the clumsy solution of both forcing your users to change system settings and forcing other programs to not have access to the convenience of the print spool. See :
PrintQueue Class
W̶i̶t̶h̶ ̶t̶h̶e̶ ̶a̶b̶o̶v̶e̶ ̶y̶o̶u̶ ̶c̶a̶n̶ ̶p̶r̶o̶g̶r̶a̶m̶a̶t̶i̶c̶a̶l̶l̶y̶ ̶c̶h̶a̶n̶g̶e̶ ̶t̶h̶e̶ ̶s̶p̶o̶o̶l̶/̶d̶i̶r̶e̶c̶t̶ ̶s̶e̶t̶t̶i̶n̶g̶ ̶(̶r̶a̶t̶h̶e̶r̶ ̶t̶h̶a̶n̶ ̶d̶o̶i̶n̶g̶ ̶i̶t̶ ̶v̶i̶a̶ ̶p̶r̶i̶n̶t̶e̶r̶ ̶p̶r̶o̶p̶e̶r̶t̶i̶e̶s̶ ̶m̶a̶n̶u̶a̶l̶l̶y̶)̶ ̶o̶r̶, (Nevermind, that property is read-only.) probably better, use something like
PrintQueue.GetPrintJobInfoCollection
to monitor the queue when sending out print jobs.
The above is supported in Server2008 and VistaSP2 and above. For XP and earlier you may need to follow something more like :
How to query the print queue on Windows
perhaps by listening for :
WM_SPOOLERSTATUS