Is there any difference from having plain text with your keyword or having an image over it for aesthetic value ?
I think link-text is weighed better. I would always prefer a text-link to a picture link if I can get one.
Huge difference SEs don't read images. Thus you won't get the benefit of the ranking for the text in question. Are we talking Back Links?
Have both, use an image link with a text link below it. The image link is just for users so "nofollow" it and use the text link for search engines.
I don't think you need to put 'nofollow' on the image. It counts as a link (probably not weighed in highly though), and even if it doesn't, google won't count it negatively either. Do put in a description on the image, so google has some idea what the link is about.
If you place your link in SEO terms, want traffics from search engine then you should use text link. But if you place your link for traffics terms, want traffic from people click on your banner when you advertised then use image (banner) because it's more attractive and outstanding then people will click more. You can also use alt tag with your images but not as good as use anchor text link.