Basically USB protocol requires one device to act as a Master, and others as Slaves. Master-master connections are not supported. Master devices typically have A-type connectors: computers, photo frames that support external USB media, etc. Slaves are your typical USB sticks, mice, etc. Master supplies power to slave over USB bus.
To cloud the water, USB OTG specification allows devices that support it to behave both as a master and as a slave (some Android phones, etc.).
Your photo frame is a master device, you need your Raspberry Pi acting as a slave. As far as I know, it is rather tricky, if possible at all: As a computer, Raspberry Pi supports USB master mode, but apparently USB slave mode also should be possible on some devices. Quoting the forum you mentioned in your question:
The model B has a built in 2 port USB hub, which does not know how to act as a USB device. The model A directly connects the processor USB interface, which can be configured either as a USB device or USB host.
Obviously, that won't work until people have Model A's to develop on, and someone has the talent, the time, the inclination, and the documentation, to write the device driver.
Bit-banging a slow speed USB device over GPIO should be possible on the model B.
This all will require some very significant hacking, many hours of programming/debugging time. In my opinion, waste of time.
My suggestion: Throw out your photo frame, get some second-hand/cheap small LCD display, connect it to your Raspberry Pi and use it as a photo frame. Much easier, faster, etc.
Windows 7 performs a consistency check when USB storage is mounted, though I don't know the exact details of the process. I have a couple thumbdrives that I sometimes mount in Linux that always get a little hiccup from Windows 7 the next time I move them over after messing with files.
Best Answer
IGRS Linksee USB 2.0 802.11g WiFi Wireless Remote Mass Storage Dongle also seems to achieve what you are after. Unfortunately, I have not yet seen a solution that is platform independent.