If you're making a test for widespread distribution, remember that different computers have different display sizes. Check your material on different Windows screen resolutions and if you're using the web, check it on different browsers. Check that the reports you get are what you want before you deliver the test to lots of people. Try it out first with a few and check the reports. You may find you need to adjust the authoring details in some way to make the reports better.
Graphics are great for making a computerised test come to life.
But if you use graphics, don't use more than 256 colours unless you need the resolution. It's usually best to cut them down to 256 colours to save space or download time. Please, please, please make regular backups of your data. Make copies of tests, questions, graphics and other files to floppy disk or tape every day they change. Last but not least, our sister company, Question Mark Corporation, have made an excellent glossary of terms used in assessment.