MythImage: fix loading images from slave BE's

52fa7ecc added a check in MythImage::Load() to make sure the file exists but
unfortunately RemoteFile::Exists() only looks on the master BE so trying to
load an image on a slave would always fail.

Just do a quick fix using RemoteFile::FindFile() for now. The proper fix would
be to fix RemoteFile::Exists() but that would require more intensive testing
to make sure fixing it doesn't have some unexpected side-affects.