It is probably hard to imagine this being a bed from this image, but keep in mind that this structure would have been decorated with pillows, sheets and somewhat of a mattress. But what makes this a definite bed is for a start the building is a brothel, so therefore beds have to be plentiful. Also from the positioning of the bed up against the wall makes it even more recognisable as well as the elevated end for a head rest (keep in mind this would have been more comfortable with cushions). It is also possible that slaves in really bad working conditions weren't given a mattress if only a really thin one! Notice in the source the elevated end of the bed as seen in your attached image

Source: https://s-media-cache-ak0.pinimg.com/564x/dc/4b/b5/dc4bb5f86ca001e95b4eee8e4c1a4637.jpg

Roman Housing, Simon P. Ellies