"Catalog index file created on Canon digital cameras; stored in the CANONMSC folder and corresponds to other folders saved on the memory card; i.e. "382.CTG" contains information about images in the folder "382CANON."
CTG files contain information about the number of images stored in each folder on the memory card (typically up to 99). They should not be manually opened or edited by the user. The camera will automatically delete the CTG files when the corresponding images are erased.
CTG files are much smaller than .JPG files and do not contain any image data. Therefore they cannot be converted to JPEG files."
Here is the source: http://www.fileinfo.com/extension/ctg
You looked for your images in the wrong folder that's why you see strange file.
On Canon image files are located under:
\DCIM \xxxEOS1D \xxxCANON \***YYYY.JPG \CANONMISC (DCIM control metadata) \MISC