There will be plenty of visual material presented in the social studies, math and science tests. The business section of any major newspaper is usually replete with bar graphs, circle graphs, and statistical tables. Don't forget that political cartoons are often included in the Social Studies test. Encourage your class to bring in samples of political cartoons from the local newspapers so they will know what to look for and the meaning of symbols. For example, the US is often portrayed as Uncle Sam or an eagle.
Many math workbooks help students create their own bar graphs, tables and flow charts.
The GED books have plenty of examples, too. Many of them think you know everything there is to know about any subject. If you make a mistake on the board, your students will happily pounce on you to point out your error. Do not panic. We learn from our mistakes, and more importantly, they will, even if you feel it is at your expense.