mantener la alta imagen que los clientes tienen de la marca X
-->
sustain/maintain/uphold the respect that clients have for X

good luck!

This is image the marque in French:

image de marque (f)
n. brand image, reputation of a particular manufacturer

ok

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Note added at 1 hr (2008-09-26 02:32:44 GMT)
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maintain the high image the clients have of...

Διατηρείς υψηλό προφίλ..

English will often use a compound term ("high-END") where Spanish uses a more simple term ("alta").

In the context of marketing, the term "high-end image" is much better than "high image."

Some examples below:

"'We want to maintain our brand's high-end image in the European and US market,' Mao said. Mao and his staff members think their fashion..."
http://www.chinadaily.com.cn/hkedition/2008-04/22/content_66...

"Can RL {Ralph Lauren} go mass market via this non-brand name line while not diluting its high end image?"
http://www.cnbc.com/id/23273084?__source=RSS*blog*&par=RSS

"Although they do not have the high-end image brands as Mondavi..."
http://content.grin.com/data/6/44474.pdf

"Cigar store builds sales on 'high-end' image..."
http://findarticles.com/p/articles/mi_qa5295/is_199809/ai_n2...

"...also gave the property a high-end image among shoppers..."
http://design.hktdc.com/suc-e615.htm

"...fabled French house's high-end image..."
http://www.time.com/time/magazine/article/0,9171,1640394,00....

"Wrung Division always provides its revisited classics while adding up new flavors, mixing quality with a tight high end image, which make all the brand's mass appeal..."
http://www.wrungdivision.com/brand/

"On Creating a Premium Brand: ...In general, when a brand has a high-end image but supplements that image with high innovation, a premium brand can be built..."
http://www.brutalclarity.com/index.php/weblog/more/on_creati...


NOTE: The hyphen is optional, depending on the style you are using in the rest of your document:

"high-end image"
"high end image"



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Note added at 1 hr (2008-09-26 02:59:23 GMT)
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More examples, this time with the entire target term "maintain high-end image" plus the term "brand":

"These campaigns will invigorate lagging sales and maintain the high end image of Starbucks brand."
http://www.mendisbrandy.com/blog/labels/starbucks.html

"Though always careful to maintain the high-end image of the brand, Michelob has introduced more line extensions than almost any brand in the beer business."
http://www.beveragenet.net/bd/1997/897/897bee.html



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Note added at 2 hrs (2008-09-26 03:21:40 GMT)
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TYPO:
More examples, this time with the entire target term "maintain high-end image" plus the term "brand"

SHOULD BE:
More examples, this time with the entire target term "maintain the high-end image" plus the term "brand"

my ideas :)

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Note added at 54 mins (2008-09-26 02:11:14 GMT)
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image = confidence

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Note added at 2 hrs (2008-09-26 04:01:33 GMT)
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I know this is hard to take but it's the idea :)

'High' does not really belong in the translation and so literal translations do not work very well - INHO. A couple of answerers have already detected this.

The meaning is better communicated by simply referring to a 'quality image'.

I prefer uphold to maintain...and I agree that high is too literal.