Kicad: Switching front Silk for Back Silk
The GerberRender.exe utility that is bundled with FreePCB generates .png files from Gerber files. It takes a --mirror argument, and has options for resolution and anti-aliasing. You can go straight from your Gerber to a crisp 1200 DPI image, flipped the right way.

Note that what you're calling "front" is actually called the bottom copper. Bottom artwork is the one that doesn't need to be mirrored if you're doing some kind of transfer method, like laser printer toner. (Of course, it gets mirrored by the printing process, so if there is any text in a bottom layer, it must be prepared backwards in the original CAD. Your CAD programs should automatically mirror bottom text; if not, you better tell it to.)

PCB fabricators understand how to treat the layers. You don't have to draw mirrored layers; only to identify which of your gerber files are top, and which are bottom, or internal layers, according to an agreed-upon naming scheme.