I'd rather see a discrete '1 month free' than a blatant picture, but that's just my opinion. I agree that it's absurd when people write '1 week free' etc.. But why not just make a rule against that? Or make a rule that the shortest amount of time you can put in your signature is 1 month?
Also, if the bracelets are coloured, won't it be obvious what it represents? To me, it is exactly the same as seeing, '1 month free from si' written down, maybe that's just me.