I, too, live in Spain and am a volunteer transcriber for FreeBMD which is not a problem because they just send me the scanned images of pages as e-mail attachments, which I then send back to my syndicate supervisor for checking before uploading to the FreeBMD site - it's all quite simple and easy to download the necessary software.
I'm still a novice at transcribing myself, but probably like you felt I wanted to give something back instead of always using the services of these wonderful sites.
At the moment my particular syndicate is transcribing the first quarter of 1907 for marriages.
If you want any more information let me know

Jacki