Programmatically extracting slides as images from a PowerPoint presentation (.PPT)

You should probably give unoconv a try. According to the man page, "unoconv is a command line utility that can convert any file format that OpenOffice can import, to any file format that OpenOffice is capable of exporting. "

So, to convert ppt to, say, png, you do:

unoconv -f png some-slides.ppt

Should that failed, you may try JODConverter or PyODConverter from Art of Solving. For example, you can use JODConverter from command line:

java -jar lib/jodconverter-cli-2.2.0.jar document.ppt document.png

Given your requirement to run on Linux, it's probably easiest to automate OpenOffice.org.

Exporting as HTML will give you a JPEG or PNG image with configurable quality for each slide.

Years ago I used Slide Splitter for Impress for this same exact need. Worked with ppt slides as well and exporting to .jpeg.