How to build and run a data-generating tool in a Visual Studio build?

Question

My main project builds some software that requires a data file ('image' below). This image is generated by another program that I can build in another project. I would like to automate all this in one Visual Studio (2008) solution.

In summary I have the following dependencies:

1. software.exe should only be run if image exists
2. software.exe depends on software.c (but not image)
3. image depends on tool.exe and other data files
4. tool.exe depends on tool.c

I have tried adding a post-build event in tool.vcproj, which runs tool.exe. It kind of works, except that it doesn't automatically re-execute tool.exe if the image is missing or if the other data files have changed.

Suggestions?