typoscript - TYPO3: Assign altText to dynamic header image -

i using typoscript snipped render header image first image assigned in page's resources:

    headerimage = coa     headerimage {        10 = image        10 {             params = class="logo"            file {                import =  uploads/media/                import.data = levelmedia:-1, slide                import.listnum = 0                width.field = imagewidth                width.wrap = |m                height.field = imageheight                height.wrap = |m                treatidasreference = 1              }         }     }  

how set alttext that coming image's properties, not fixed value set alttext = xyz ?

cheers