Since TYPO3 6.0 the media field uses File Abstraction Layer (FAL) to create references to uploaded objects (e.g. images). Because of this change, you can't use the IMAGE content object to fetch the inserted object, but first have to use the FILES content object.
For you case, the TypoScript should look like this:
after.cObject = FILES
after.cObject {
references.data = levelmedia:-1, slide
references.listNum = 0
renderObj = IMAGE
renderObj.altText.data = file:current:description
renderObj.file {
import.data = file:current:publicUrl
width = 250c
}
}
References: http://docs.typo3.org/typo3cms/TyposcriptReference/ContentObjects/Files/Index.html