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Fracois Brutsch,
Thank you for your research as this allowed me to find a method to put an image in the form header description!

Try the following:
In the associated spreadsheet:
Insert->Image then paste in the full URL of a web based image (this can also be the URL of an image in a published Google Docs text document).
Publish the spreadsheet, open the published spreadsheet and copy the URL of the image, this new URL will look something like this:
http://spreadsheets.google.com/pub?key=<spreadsheet key>&oid=1&output=image
e.g. this public image in a published spreadsheet:
http://spreadsheets.google.com/pub?key=tN4TW0lAFR_xcD6ktHLzMuA&oid=1&output=image
Now copy the bold part of the above URL and go to the form editor (Form->Edit form) and put in the Form header description:
<img src="pub?key=<spreadsheet key>&oid=1&output=image" >
or using the example URL:
<img src="pub?key=tN4TW0lAFR_xcD6ktHLzMuA&oid=1&output=image">

Now save the form.
Next go to the live form (Form->Go to live form) like this sloppy test one:
http://spreadsheets.google.com/viewform?hl=en&formkey=dE40VFcwbEFGUl94Y0Q2a3RITHpNdUE6MA
And note the inserted Google Chrome image at the top

FYI: the full header description looked like this, showing a number of tags Gill and I found one could use - other than the <img> tag you helped to get working:
What <b>is</b> <i>this</i> <u>for</u> <a>a</a> kind of form? <img src="pub?key=tN4TW0lAFR_xcD6ktHLzMuA&oid=1&output=image">www.google.com <b>www.google.com</b> <i>www.google.com</i> <u>www.google.com</u> http://www.google.com<b>http://www.google.com</b> <i>http://www.google.com</i> <u>http://www.google.com</u> <b>test</b> <i>test</i> <u>test</u>


Summary: You can insert <img src="pub?key=<spreadsheet key>&oid=1&output=image" > in a Google Docs form header descriptionprovided the image is part of a published spreadsheet and the URL of the image taken from the published spreadsheet looked like this:
http://spreadsheets.google.com/pub?key=<spreadsheet key>&oid=1&output=image

Note of course this image can be taken from any published spreadsheet, not necessarily from the spreadsheet associated with the form!
Note 2: the <img src="pub?key=<spreadsheet key>&oid=1&output=image" > can also be used in the thank you page (in the form editor ->More actions->Edit confirmation.