A byte in hexademinal number is an alphanumeric number up to two characters long.  It uses numbers 0-9, and letters A-F [basically 10-15]. keeping things simple, the largest byte would be FF which translates into 255 (16*15+15).  In the 256 ascii system, there are characters associated with these values.  You may quickly pull up an ascii table on google, but you might find one that has 128 characters instead.

Images we tend to think of in binary (base 2) or hexademical (base 16) data.  And an image contains a lot of data.  But this data can also corresponds to ASCII character data (0-9, a-z, A-Z, punctuation, etc, etc).  The steem/hive block chain doesn't like the ASCII encoding system, and will reject certain characters.  So there exist other encoding mechanisms besides ascii.  You use a base 58 encoding system every time you enter your keys for example.  A base64 is an encoding mechanism that uses 64 characters, but it takes more space to save data [then binary] but it gets past hive/steem's limitations.  After the encoded data is save, the data has to be decoded to return it to its original state.  There may be additional steps depending on what the creators intent is.

There are still other limitations. The maximum size of a steem post is 64k (64*1024=65536).  but due to using base 64k encoding, it shrinks it down to about 48k.  Custom jsons were limited to 2k, but they may be up to 8k now.  custom jsons won't appear on a steemit type frontend dapp, but takes many times longer to store.  A really large post on a front end dapp could end up being pruned for size, but there are ways around this limitation for dapps.

There are images, books, even archived websites on the hive block chain.

  When the Justin Sun steem witnesses talk about limiting the number of post that can be made a day, or saying only steem should control dapps, in their effort to purge it of voices it doesn't like-they are shooting themselves in the foot.

  Although to prevent cp from getting on the chain, we (not nova) limit the size of the image store to the chain.  Here is an example of an archived website on steem. The parsing isn't perfect yet, as somethings may still load from nature.  Really waiting for the front end guy and a new chainid.  

https://steemsafe.net/php/loadfile.php?fileid=620&index=0

You seem to have found it, yet for others who may follow the link is...

https://hive.blog/base64/@novacadian/base64-encryption-explained