From a TCIA user:
I have managed to locate and download the diagnostic whole-slide images I was searching for, partly thanks to the Cancer Digital Slide Archive http://cancer.digitalslidearchive.net/ which also has TCIA info.
The answer is that the diagnostic images are indeed on the Legacy Archive at the Genomic Data Commons website, just as they should be.
If the user selects to search by File identifier, then the image file can be located.
For example, if the patient ID is TCGA-A6-2672, and one searches for the file containing the string TCGA-A6-2672-01Z then the image file (in SVS format) can be added to the user's cart and downloaded.
The suffix -01Z is important: if not used, the image file is not found by the Legacy Archive Portal.
Unfortunately the diagnostic images (as opposed to frozen images) are not linked directly to patient IDs, so searching directly by TCGA-ID does not reveal these images.