Cant get movies to play on Toshiba TV


I have the WD TV player, and I have a 9 year old Toshiba 42 inch TV, and the WD 2T drive.  When I plug everyting up i can see the display screen for the WD player on the tv just fine, and I can search the drive and see all of the movies listed there (mostly mkv and iso files). But when I try to play I get the error message as follows, “unable to play the selected file. please refer to the user manual on how to change the file to a supported format”. I have done an error check on the WD drive, I have done a reset on the wd player, and just to make sure the files were ok, I downloaded the free vlc player and can play all files on the WD drive on my pc. BUT i cannot get them to play on the TV.  Also- there are a few jpeg files on drive which will not play on the tv, and a few pictures that will also not play on the tv.

Any suggestions?