I don't think all are jpg's. Only some of the materials that require a specific texture use a jpg - for example most of the wood materials have an image file applied during the render so you can see the pattern. Material library of jpg and png files -
C:\Program Files (x86)\Common Files\Autodesk Shared\Materials\Textures\3\Mats. (Path is Windows 7 based and assuming RME 2012)
As for transferring projects standards - not sure if that would take texture files from another model and add them to your library - assuming the problem material is from a linked model. I'm not 100% sure that this is your root cause but seems the most likely place to start. May be worth opening the various models and purging unused materials to help locate the offending material. I have checked the above by removing a png file from the above folder and rendering and I get the error attached.
Either that or the path to the material textures is not correct.