To collect evidence of an image on Instagram, capture the picture and who posted it. The username of whomever posted the picture will be above the picture.
If the abuse or harassment was in the comments, take a screenshot or screencast of the username, the image, and the comment. Include the date and time that is at the bottom of the comment. Also make a note of the date that you took the screenshot or screencast.
You can also capture information from the profile of the person who is harassing or abusing you. Tap on the picture of the person to bring up the profile. The profile will show the name on the account in addition to a larger image of the profile photo. Take a screenshot of this information. Include the web address or URL of the profile by clicking on the 3 dots next to the user name. You can copy and paste the profile URL into your notes.