I don't know about this solution, but I haven't played Reforged just yet. I believe changing an animated background in Reforged to a static images seems to be difficult to execute, since Reforged had always use an animated background menus that was saved with WEBM format (instead of MDX).
An interesting fact: You can also switch between ROC and TFT animated background menus with the latest patch of Reforged by clicking on the game logo (it's located in the right panel where you can find the options to play Single Player, etc.). Thanks to deepstrasz for providing this easter egg.