You need photo editing software that supports layers and blend modes, like Photoshop or GIMP. If you want to create, say, a tourist guide, then you place a photo of the sight the code is supposed to link to on layer 1. Put the qr-code itself on layer 2 and apply the “negative multiply” blend mode to the layer. Check with you mobile phone if the code works (might not work, due problems in contrast). Export and done.
You can create qr codes online, using e.g. http://qrcode.kaywa.com/
See also: How to create a mobile city guide