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TSConfig:

##################
##   IMAGE FX   ##
##################
TCEFORM.tt_content.image_effects.addItems.30 = Runde Ecken

Typoscript:

## TYPO3 6.2.x

temp.image < tt_content.image.20

tt_content.image.20.1.stdWrap.cObject = CASE
tt_content.image.20.1.stdWrap.cObject {

	key.field = image_effects
	
	default < temp.image
    
    30 < temp.image
    
    30 {
    	1 = TEXT
    	1.params = class="corner"
    }
}


## TYPO3 4.5+

tt_content.image.20.1.params.cObject = CASE
tt_content.image.20.1.params.cObject {

	key.field = image_effects
	
	default = TEXT
  default.value =
    
	30 = TEXT
	30.value = class="corner"
}

#########################

## TYPO3 4.5+ und 6.1+
## gilt für alle Images (aktuell bewusst auskommentiert)
#tt_content.image.20.1.params = class="corner"


Quellen:

Thread zu 4.x:
http://www.typo3.net/forum/thematik/zeige/thema/102054/

Blogpost zu 6.1.x:
http://blog.andreasbecker.de/2014/02/05/typo3-6-1-eigener-bildeffekt-hinzufuegen/