Icons, not simply as "pretty pictures", have been an aid to prayer for centuries. The Orthodox Churches are most famous for this but it is also a part of Catholic tradition.

For the Orthodox tradition Icons are not "Holy Pictures" but a visual Gospel, a way of glimpsing the Divine in art. Hence Icons are not just painted but works of prayer and devotion with a language of their own. More information on Icons can be found at http://www.iconsexplained.com/

Some people do not use Icons but pictures, or even just a simple candle or Cross as an aid to concentration. They can remind us of why we are praying, what we are praying for and also give something for the eyes to focus on to "free" the mind to focus on a meditation.

Many Catholics light candles in front of a statue as a sign of their prayer ascending and spend a few minutes in prayer. This especially true of statues of Our Lady, look at Lourdes for an example of this.

Basically, if an image or even an object, helps us to concentrate on our prayer it is good and an aid, if it becomes more important than the prayer then the priorities are wrong.