However I think you can also disable image ads on Mellowads? Don't quote me here but I remember seeing text ads from them. Not sure if it was an error or the website has the ability to choose this. Also, they do have network campaigns - not just site-specific.
You're wrong on the reason. Text gets more clicks when used properly (contextual). Banners are used because they do branding. CPM advertisers don't care if you get 1 click or 50 million clicks, as they are paying because they get that branding in front of you. The whole point behind it is that when you think about something you need/want, you will remember an earlier ad and choose their brand.
I really wish some of you knew what you were talking about. There's so much misinformation spread here, it's painful.
Banner CPM campaigns can be made for branding purposes, but 95% of them are just there to get clicks. Branding is just a nice extra - they are there to get results.
I think you are confusing "contextual" with text ad CPM ads, which is what the OP wants. Those aren't nearly as contextual and, while the quality of the traffic will be higher, the advertiser will receive way less traffic with the same amount of impressions, given they use a banner with a decent CTR.
Next time you try to be a smartass, choose wisely.
I agree that banners generally get more clicks than pure text ads when both are unrelated to anything.
Also just realized it looks like you're an advertiser, rather than publisher. I was on the other team, :p.