Questions and postings pertaining to the usage of ImageMagick regardless of the interface. This includes the command-line utilities, as well as the C and C++ APIs. Usage questions are like "How do I use ImageMagick to create drop shadows?".
I know this thread is very old but we encountered the very same problem and I wanted to avoid the "wisdom of the ancients" effect.
It looks like that many images can be passed to convert via STDIO only when they're in PPM format. If you don't want to change your app to output PPM you can pipe your app output to ffmpeg and then to convert like this:
Extra bytes at the end of a PNG stream are ignored by IM, hence only the first PNG is processed. The same is true of JPG. Some formats (eg PPM, GIF, TIFF and MIFF) can contain multiple images, and each will be processed.