Hello eye,
Basically, anything created in Maya, that is converted to bitmap image will be placed in the sourceimages folder, exept rendered images and IPR files. If you convert to solid texture a 3d shader, it will be considered as a source image, applied onto a model. But if, for example, you select a shader, and export it from the hypershade, it will be saved in the texture folder. That's it ... You should also refere to the Maya help file, or to the paper doc if you want it to be explained in simple words ... Can't do better. Anyway, hope this helps 
Alban