In the README, the example for loading from a stream is:

function load(f::File{format"PNG"})
  open(f) do s
  skipmagic(s) # skip over the magic bytes
  # You can just call the method below...
  ret = load(s)
  # ...or implement everything here instead
  end
end

# You can support streams and add keywords:
function load(s::Stream{format"PNG"}; keywords...)
  # s is already positioned after the magic bytes
  # Do the stuff to read a PNG file
  chunklength = read(s, UInt32)
  ...
end

This works if the user calls load("somefile.png"), which will end up calling the load(::File, ...) method, which handles skipping the magic bytes before calling load(::Stream). What I'm not super clear on is how this is intended to be used directly. e.g. I couldn't just do:

open("somefile.png") do io
    load(io)
end

In ImageMagick.jl it looks like load is called on the IO stream directly without skipping the magic(k) bytes.

So if it seems like the way ImageMagick does it (leaving the stream at the very beginning of the file when calling load(::Stream)) I can put in a documentation PR that clarifies that.