I guess, you probably need to pick up randomly two pictures from the same set, so that the two pictures are always different. For this, you can use the Roulette Element in the following manner.
Add new Roulette Element and set it up to produce a random block with all picture numbers. Normally, Roulette would draw a single number from this block on each trial. What you need is to draw the whole random block and pick up any two numbers, for example, first two. After that you command Roulette to regenerate the block for the next trial.
In order to do this, set a proxy variable to Random Values Property of the added Roulette (not to Random Value, as usual!). The random values property returns an Int32 array containing the whole block of random numbers. Set a proxy variable to “ForceNewBlock” property of the same Roulette. In the trial onset snippet (e.g. “After Onset”) write the following code:
The above code would draw two different random numbers on each trial that can be used to select pictures. However, if you a need balanced randomization of two pictures in many trials, something more complicated need to be implemented.