Cronos,
Setting an attribute on all graphics in the document should be dead easy as part of the conversion table. And even if it is not handled by the conversion table it is going to be much easier after conversion. So I would not worry about the alignment of the images in the original documents at all. There are other ways to spend your time. I would expect lots of problems that cannot be handled by any conversion table. You will probably need to do some scripting in pre-processing and post-processing to make the materials come out as valid DITA with minimal manual afterwork. One tool you will have to purchase is FrameSLT, by West Street Consulting. At only $100 it gives you full XSLT power (and more) within FrameMaker. I would not be able to survive my projects without it.
I specialize in this work - automated conversion from unstructured to structured FrameMaker (whether it is DITA or something else does not really matter that much to me). I am not doing this for free, but if you get stuck drop me a line and I can see what I can do for you. Thousands of unstructured documents to DITA sounds like a job you could use a very high degree of automation for.
Good luck
Jang