Normally, when you have completed gluing your pictures or photographs on to the surface of a decoupage project, you apply varnish or sealant on it. In order to strengthen your decoupage further you can also apply wax on it at regular intervals. It’s recommended not to keep it in direct sunlight.
With Decoupage you should brush the varnish in a way that the brush strokes are not visible; instead the varnish should simply flow. The acrylic ‘mod podge’ does not flow, so instead of using that you can use acrylic varnish. No extra ordinary talent is required to do one. Simply learn a few tips and start putting in your innovations. Note that if you purchasing your resources from the online vendors, be cautious of poor quality/frauds and see to it that there is some refund guarantee.
When you use photographs and images for it and need a rich black effect on the image, you can do so, using a color photocopier. The percentage of enlargement or reduction can give the image, a symmetric design. So you can also consider this option for the effects of your choice.
Always lay out the photograph to create the effect you desire, before applying any glue...