To add images or videos or any other large or binary file type to an activity you can either use our enrichment process or for images you can use our image API.
Enrichment: Create a new JSON field in the activity and have the value be some kind of reference to that media. For example, you could put a URL to an image.
When you read the feed back you'll enrich it, and resolve any references you've stored. This is also a great way to not send PII or other sensitive data to Stream but still maintain the integrity with the activities.
Image API: Check out the docs and you can also use our built-in image manipulation capabilities as well as available CDN. (additional fees incurred)