Is there a way to use Stellar, combining multiple transactions and their memo fields, to store simple data like a binary image or a qrcode?
Stellar allows you to store data on the blockchain in two ways:
Attach data to an account: you can attach key/value pairs to an account. Each key/value pair you attach to an account raises the minimum account balance by 0.5 XLM. This is in place to disincentivize people from storing large amounts of data on the blockchain.
Attach data to a transaction: you can include data in a transaction via the memo field. There are four types of memos you can include:
a) MEMO_TEXT : A string encoded using either ASCII or UTF-8, up to 28-bytes long.
b) MEMO_ID : A 64 bit unsigned integer.
c) MEMO_HASH : A 32 byte hash.
d) MEMO_RETURN : A 32 byte hash intended to be interpreted as the hash of the transaction the sender is refunding.
The easiest way to attach a binary image or a qr code to a transaction would be to store it in a distributed database such as IPFS and take this hash and attach it to the transaction via a MEMO_HASH.