At the end I'm printing normally from the browser. I contacted Brother Spain, they said they were going to give me a solution, but they haven't answered anything yet...
I was able to print using this printing using the Base64 example that comes with jZebra, but after printing the image, the printer doesn't cut the paper and keeps pulling out the paper, I have to shut down the printer in order to make it stop.
I had to modify my browser's printing page properties, remove some margins... Make sure is set to print vertically... But it "worked" with Base64 a little bit.
But at the end I just went to the configurations of the printer and selected to cut the paper after the info was sent (there are two options; cut the paper at the end, and cut the paper after receiving the info) I'm translating from Spanish so probably there are other words in that phrase. So after configuring my browser, I'm using the normal javascript printing function and it works fine (I open a separated windows containing the barcode and the print dialog pop ups telling how many images to print.)