upload base64 encoded image data

[hh]

Because "+/=" is possibly in the encoded data:
Did you already try the following?

put urlEncode(base64encode(myImg)) into tImage64
put quote& "data:image/jpg;base64," & tImage64 &quote into myContent
post ... "content": & myContent ...

The image size will increase by a factor of 3 to 4 using base64.
There is also a server side post_max_size, but this is usually
announced (some online converter have this around 20 MByte).