This only works through the mobile app on a smartphone. You cannot do this from the webpage, or on a chromebook. Download the Google Classroom app for your smartphone (Free) and sign in with your student's full email address (Network ID). Any classes your student has joined will show up once sign in is complete. If you are signed in with a personal account, you can tap the three lines in the upper corner to manage accounts used by the app.
Sharing a Photo & Comment with the whole class
(only if the teacher says this is OK.)
Make sure you are in the stream tab at the bottom.
Tap on "Share something with the class"
Tap the small Paperclip Icon to attatch a photo.
Tap "Pick Photo"
Tap the photo you want to send.
Wait for it to upload
Tap in the text area to enter something you want to say with your picture.
If you sent the wrong picture, you can delete it by tapping the X next to the photo's icon.
Once you are done, you can tap the Paper Airplane button to send it. Again, this will be visible to all students, so only do this with the teacher's permission.
It's now been posted to the whole class.
If you made a mistake, tap the three dots next to your post.
Then tap delete, and start over.
Sharing a photo while turning in an assignment.
The assignment should be completed by the student first, as this is done at the last step where the student is "turning in" their assignment.
In the Classwork tab at the bottom, find the assignment that requests submitting a photo.
Tap "Add Private Comment"
Tap "Add Attachment"
Tap Pick Photo
After choosing the photo to send, and it finishes uploading, you can tap on "add private comments" to write something that only the teacher will be able to see.
Note, that once you click, "turn in" you will no longer be able to edit, or access the assignment itself, so this should be the last step.