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In the text box properties general tab set the representation option to Image
You can't use an expression to specify a background image for a text box using the Background->Image option.
What you can do is load the images with an INFO (or BUNDLE) load into the qlikview document itself and then reference the images stored in that table. See this document for how to do that: QlikView Technical Brief - Loading Images into QlikView.zip
In the text box properties general tab set the representation option to Image
I do agree with Gysbert that this is not the best way, it would be useless on any other computer with images on your C drive. But it is possible to display the text/link from a textbox as image (there is indeed no expression for the background image).