All I can think is that at some point you've added assets to a menu screen and removed them, but that DVDSP has retained the fact they are there. This happens occasionally - have you tried deleting the 'PAR' folder for the project? (look in your prefs to see where these are kept)...
Secondly, remove the JPG from your menu - they end up looking very ugly when compressed to MPEG2. For better results save your menu background as pict or even better, as tiff. These are lossless formats, and convert to MPEG2 better than jpeg does. When you replace the image, change the name of it and DVDSP should 're-parse' the project, re-create the PAR files and update it's reference info.
This *should* stop the error... but if not there is something further amiss.
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lafcpug mod