OK. I changed all my ads to text only. I just didn't know how to do it.
On the "Ad Units" page, I checked the box on the left at the very top of the page, which enables the "Action Box".
Then I checked "Text Only" and Voila - all done. No more "Download" ads. No more misleading "shock" images. No more Flash.

Now, if only changing ad size globally were as simple.
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Changing ad types/sizes *is* easy if the ads are located outside of content, e.g. on the sides of the page or at very top or bottom. It's when the ads are embedded in content that the resulting empty space is evident.

You can use a simple JavaScript to switch different <div> containing different ad types. I use SSI to call the specific ad code and fill the <div>. If interested, do some internet searches for "JavaScript to switch ads" to get other ideas.

Further discussion on how to implement these methods should probably be done in the Webmaster General [webmasterworld.com] forum.

You'll have to let us know the results of your experiment. I'm curious what effect it will have for you.

I agree though. Adsense's own suggestions are, most times, nonsense.

I sure wish they'd let us upload our ads (like you can do with DFP, I think) via spreadsheet - for those of us with multiple sites and tons of placements and channels, that would be a godsend. I mention it every time they send out a survey, but of course, crickets.

Keyplayer - Unfortunately, I am not a coder - self-taught html is about it, plus what I read here.
Divs and javescript are over my head, and most other things. But I will now be reading daily in "Webmaster General" now, as well.
Thanks for the hint.

Early this Monday morning ( For testing, I switch things at midnight Sunday, so I can get a nice weekly experiment time period.)
I consider weekly from Monday through Sunday, not Sunday through Saturday (calendar) - makes more sense, because it doesn't split the weekend.
After 11 hours into Monday morning, it does not look good. It is as if someone turned off the faucet.
Earnings are WAY down, far more than I would expect with the changes I made (all text only, all 250x250s), for my top 10 pages.
Reports indicate that the text ads always pay more than image, and 250s pay better than 300x250.
So, it's hard to say what is really going on. I will report back after 1 week.
At least the 250s are showing up, whereas before, the 300s often were absent from MY browser (FF).

Also, my mediadotnet ads are totally absent. I emailed them, and they said everything was fine.
Earnings are good, so this must be my problem, Can't see ANY of their ads on Chrome, FF or Safari. Weird.
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If you block image ads and only run text, then you are reducing auction pressure. Text ads were paying better than image ads because they were competing with image ads. Once the competition is gone, text ads revenue will drop because advertisers do not have to pay as much to show them.

Along with what's been said, running the report Advanced Reports >> Ad Types shows me that, at least for my sites, Text Ads earn WAY more overall compared to Image/Rich Media ads BUT the CPC on Image/Rich Media ads are nearly twice the value. So, I don't think you can show one without the other and hope to have better revenue numbers unfortunately. You're better off leaving both on unless some day there is a massive shift in the ad industry.

@ember
Ah, so have a made a big mistake?
I know it can be a problem changing more than one thing at a time, to know WHICH change had the effect.
250s have always paid the best, for me, so I am thinking it is not that.

So, what is your opinion?
1 - One should NEVER go all "text only", ever, for any reason, even to avoid the massive pile of cr@p that is "image ads"?

Should I admit defeat, and immediately switch back to ALL "Image plus text"?

EDIT - Couldn't wait. I switched back to "Image plus text", to try and stop the bleeding. The 250s stay.
Thiis will provide a much cleaner experiment.
Thanks.
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I would run both text and image to keep the auction pressure in place - it is all about supply and demand - but would block the worst individual image ads. Not all of them, just the ones you really can't stand.