My big advice about visuals, boiled down into the tiniest format possible.
– Know your business's personality
– Know your target audience
– Draw visuals that communicate your personality to your target audience

And, alongside all that, keep it really simple. Small businesses don't have time to be graphically clever or tricky – you need to create simple graphics that work to get your message communicated to your target audience. You don't want to confuse them or to force them to work it out in their minds, because you'll lose business that way.