How can I post photos to my iPhone Instagram app from Google Drive?
To share a photo from your Google Drive to Instagram on your iPhone, you open the photo you want to share (by tapping on it), on the top right you'll see 3 little dots, tap the dots, and you'll get a list of options.  Choose the 'Open in' option and that will give you the option to share with Facebook etc.  if Instagram isn't there, go to the More option and enable it.  Hope that helps - took me ages to work it out!