Second only to fonts, images are the most consistent source of trouble with a PDF file. The Images category (Figure 13) lets us catch many of the problems associated with a document’s images.
Figure 13 An awful lot can go wrong with images; you’ll want to do some careful thinking about these controls.
The most common image-related problems have to do with image resolution (too high or too low) and compression (wrong kind or not at all). You can also detect whether images have been placed in the PDF file as OPI (Open Prepress Interface) placeholders. If you aren’t familiar with OPI, the short version is that it lets an intelligent printing system replace embedded images at print time with high-resolution versions. The shorter version is that if you aren’t familiar with OPI, then you really don’t want it in your PDF file.
As you can see in Figure 13, my eBook profile will report images with resolutions below 36 dpi and lossy images (e.g., jpegs) as warnings; these are probably alright, but I want to see them. Images that have too high a resolution, are uncompressed, or use OPI are reported as errors; all of these will make the PDF file much bigger than I want it to be.