Would it make sense to go for image only ads?

I don't think so. Select the setting that allows for either type ad to appear.

That way you'll get the better paying ads whether they are image or text.

If you select image only you will get nothing in many instances. The inventory of image ads is not as large as the text ad inventory. In some areas, there aren't any available....

Do image ads typically pay more than text ads?

Not necessarily. Both participate in Google's ad auction system.

Image advertisers might have to bid higher than text advertisers because they will be filling up an entire adblock, but that doesn't mean that your income from image ads will be higher.

Unless you have very strong aesthetic or moral reasons for wanting only image ads or only text ads, you are best off allowing both.