I would suggest a combination of both, Text ads are adequate but sometimes people won't notice those ads, thus limiting it's chance to be clicked. Having a full image display ad is also not recommended because people now tend to get discouraged when they see huge ads blinking or taking their attention instead of what they really looking for. A mix of both would be advisable because it's not too flashy, nor too little. And the placing of ads is also has an important role mind you.