There's likely no way you can do exactly what you're asking for. The TV reads from a mass storage device, which it expects to never change, unless it writes to it. In other words, it doesn't know when it needs to reread the file allocation table to find a new file. Changing the file system behind the scenes may lead to a situation where the TV never discovers the new file, or worse, crashes because of the inconsistency. The USB device could still simulate an eject/reinsert cycle to get around this problem, but I do not believe there is such a device on the market.
If the TV has a web browser, you could of course use that with a small page on a cheap web host, which refreshes periodically.
But otherwise, you would need an external computer. The Raspberry Pi comes to mind. It's a small (but not too powerful) single-board computer for $25 which has an HDMI output and is powered by USB. It's small enough that you could easily attach it to the back of the TV. It could then connect to the Internet or local network using ethernet or an external USB Wifi device. This solution would probably all in all cost you around $100 in components, but would also require you to set up the Pi to open a web browser or similar, and also simple web page or other mechanism to push the images to the device. This would require extra work, but may be the best solution with regard to cost and size.