Poster work should generally be quite prescriptive, ie the task should have lots of criteria that the poster needs to address. It is not a purely artistic task.
I always have my KS3 classes do a 'title page' for the start of each new topic. I tell them that it has to have science in it - it can't just be a page of bubble writing. They need to think about what they know about the topic, and put it down on paper. If they can't draw, then a few words (usually in colour and at a jaunty angle ) is great! What is important to me is that I can see where they are at the start of the topic - and for us all to see how they have progressed by the end of it.
I usually mark them by giving house points rather than actual grades, which is easy for me, and really motivates the pupils.
When I ask them to do poster work, it is very often a labelled diagram of some kind. Labels are very important. One of my favourite poster tasks is about how animals adapt to their environment (eg polar bears being white), and I ask them to create their own imaginary animal or plant with as many adaptations as possible. When the children are creative and get the science right, I know they understand.
I'm not an English teacher, but I know that illustrating a story is a very powerful indicator of understanding and appreciation. As a Sunday School teacher, I occasionally have the children illustrate a bible passage, and am usually very impressed by their level of understanding - better than adults sometimes.
The key thing is that you have to address the learning needs of all your pupils. You can't just suit yourself and teach to your own learning style. You need to vary the styles in line with the proportion of children with each style in each of your classes.