When writing a written work that includes images, what are the techniques used to add symbolism/symbolic/allegorical meaning to a work? I am thinking that you want to add symbolic images in certain panels, but how do you insure that those images you add won't be ignored by the readers? Are there some specific techniques and conventions used by authors?
I would say that the sort of symbolic meaning you're talking about should actually be subtle, and in many cases will not be noticed by readers - especially the first time round.
In many of the best graphic novels and movies which are packed with visual symbolism, such things are easy to miss if you're not looking for them. But if you go through and do actively look for them, then you are delighted to see how many there are.
However, people will likely notice these things subsconciously and they will have an impact on how they feel and relate to the story.
So I wouldn't focus on making sure your readers see the symbols - if anything you could work harder to make them more subtle!
Some examples of how you can insert symbolism:
In Avatar, when they're in the abandoned school building, one of the characters briefly picks up a tattered copy of the book The Lorax and Dr Augustine says she loves that one, then puts it on the shelf. The Lorax was said to be a huge inspiration for the premise and theme of the movie.
In Let the Right One In, most of the movie has a bluish gray palette but bright flashes of red appear when a particular character is on screen.
You can hide symbols on inanimate objects, tattoos, books and posters in the backgrounds, t-shirt slogans on passers by, even in clouds in the sky.
I hope that helps a little.