Interfacing with MSOffice files is notoriously difficult. Producing new files can be done with a fair degree of success (e.g. Jakarta POI), but extracting data from existing files is error-prone at best. Furthermore, I don't know whether the screenshot is cached inside the ppt file; to achieve what you want to do, you may in fact have to load the entire powerpoint engine in a COM->
Java wrapper and have it render the slide on the spot.
I'd much rather have the user to attach a screendump seperately. Even if they do end up uploading a 1280x1024 .bmp image, scaling and compressing that is a hell of a lot easier than fiddling with an office file format.
SCJP 1.4, SCJD 1.4, SCWCD 1.3, ICSD:Websphere 5.1