how to get binary data from a photo type image?

hi,

I want to modify a image whit tclmagick, and I want to use the
"wandName ReadImageBlob data" to read image,

I wrote the following codes :

image create photo $newImName
set img [magick create wand img0]
$img ReadImageBlob [$newImName data -format jpeg]

but ReadImageBlob method return a error:

ImageMagick: Unknown error
ImageMagick: Unknown error
while executing
"$img ReadImageBlob [DbgNub_Do 1 {2 1874 {54355 28}} {$newImName data -
format jpeg}]"


so, how to get binary data from a photo type image?

thanks!

.