Cushing's disease in itself describes the systemic manifestations of excess cortisol from a pituitary adenoma and does not "show up" on an MRI. Rather, it's the pituitary adenoma that your docs are looking for on the imaging studies. As I have never personally examined you or reviewed your personal history and workup in detail, I cannot make a specific comment as to the abnormal lab values and their cause. What I can say is that the cortisol levels are quite elevated, which in itself doesn't necessarily point to a particular cause (pituitary ademona vs oral steroid excess etc...), but in the setting of a known pituitary adenoma, the levels could certainly be due to a steroid producing tumor.
Purple striae (streaks on the skin that look like stretch marks) are well known to be associated with Cushing's. The marks on your chest could be this or could be due to thinning of the skin which is also a feature of Cushing's. Get a neurosurgery and endocrine consult for further evaluation. Good luck.