Text ads are always the first choice when are looking to monetize your site. Publishers are always hesitant to display image ads on their site.
Text ads have certain advantages over image ads that are listed below:
1. If you choose the correct font size and color for your text ads, they can easily blend with the content of your site and make your readers feel that they are part of the content and thereby generating more clicks are readers clicks on the ads to gather more information on the topic. With image ads you cannot change the color, only thing you can do is to change its position and size. Thus, it becomes difficult to make it look like a part of the content.
2. You can display more number of text ads in the space that is occupied by an image ad. More number of choices to your reader better is the chance of enticing a click on your ads.
3. The font size of the text in the image ad will never match with your content thereby making them look like a clutter and annoying the readers.
4. People love banners but rarely click on them. A more appropriate choice in this case would be to use text ad that looks like part of the content.
This does not necessarily mean that you should stop using image ads on your site. One of the best advantages of an image ad is that it will give you more per click than all the text ads combined for the size you choose. This means a very high CTR, but you should only you image ads as a filler for left over space and preferably place it very close to bottom of the page.