Firstly go to Products and select the relevant image:


Then click Image Library: 

Then select Library: 



Within the image library window, find the image you would like to add/amend the alt tag on and click the green 'Edit' icon in the top left of the image:


Here you will be able to amend the image name and Alt. text in the fields on the left hand side:

Finally click Save to save your changes in the image library: 

And then save the product to confirm the change.