Offset flanges are not highly regarded by plumbers on this forum, but I assume there is a reason for it and changing it would be a major problem. That said, it appears in the photos that the flange is recessed below the tile. This too can present a problem. Flanges are supposed to be set on top of the finished floor and not inset. Again, you have to deal with what you have. I can't tell how far below the top of the tile the flange is, and that is the telling factor on what to use to set the new toilet. For example, if the top of the flange was even with the top of the tile, just a thick wax ring would be enough. If it is 1/4" below the top of the tile, then two rings would be used. Use a plain ring first, the on with the plastic flange. If it's lower than that, you may want to add an extension ring then the wax.