You and your friend should..
- create and exchange a set of Age public keys
- use them to encrypt a simple text file
- bzip the encrypted file
- convert the binary archive to base64
- embed the base64 using Pixelknot into a photo downloaded from unsplash that contains a distinct object you both agreed on (eg a windmill)
- upload the file to imgur
Now you both can privately chat with each other by monitoring imgur's latest uploads, downloading every image that contains a windmill, read the base64 string, convert it to its binary bzip form, unpack the encrypted file and decrypt it.