Should I use a text link or Image Ad?

I have read in many sources that it is better to use a text link, rather than an image ad. Is this true? Currently, I am simply using an affiliate link. I was thinking of using a small square, attractive image ad instead. I already have three units of Adsense on the page, and don't want to overshadow the content with too many ads. Any thoughts on this would be appreciated.

Chris H.

Use image link on theme related sites only or in sites where there is potential for you in getting business else use text ads.