Опираясь на ответ христиан, здесь полный круг:
import base64
jpgtxt = base64.encodestring(open("in.jpg","rb").read())
f = open("jpg1_b64.txt", "w")
f.write(jpgtxt)
f.close()
# ----
newjpgtxt = open("jpg1_b64.txt","rb").read()
g = open("out.jpg", "w")
g.write(base64.decodestring(newjpgtxt))
g.close()
или так:
jpgtxt = open('in.jpg','rb').read().encode('base64').replace('\n','')
f = open("jpg1_b64.txt", "w")
f.write(jpgtxt)
f.close()
# ----
newjpgtxt = open("jpg1_b64.txt","rb").read()
g = open("out.jpg", "w")
g.write(newjpgtxt.decode('base64'))
g.close()