LG .tn pictures are in most cases simply renamed JPEGs, or encrypted JPEGs, unfortunately we haven't been successful in finding any specific information about this topic. Generally speaking, if the first option is right, you could simply rename the .tn files to .jpeg files by changing their suffix. If .tn files are really encrypted you will need to access them through a software installed for your LG device. In any case, the options for tn to jpeg conversion are pretty limited.
Open TN file Open JPEG file