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.