I respectfully beg to differ Thumper. A silk screen, or the screen printing process (actually silk hasn't been used in decades) is quite the horse of a different color compared to Offset Lithography. Ink thru a stencil is not ink from a printing plate produced from a negative image. Fine art screen prints or Serigraphs tend to have a higher value due to the labor intensive process of printing each color individually after the previous color has dried, or cured. Some serigraphs may contain 1-10, 20 or more colors to replicate any given piece of art and the edition run is usually low, anywhere from 25 - 500 pieces including artist proofs. Most are limited editions with the squeegees being hand pulled or printed on a semi-automatic flatbed press utilizing a vacuum table to hold the substrate (paper) in place.
Can't tell much from the Jim jpeg but it appears to be a common offset litho poster, worth 5 or 10.00. Like Dave78 said, look for the dots, or halftone screen pattern to get technical. The paper is a factor as well. Of course it could also be a digitally imaged print from a large format ink jet printer which produces it's own mechanical print pattern evidence.
It's a nice B/W drawing rendered with relative art skill but if it was an original, it probably wouldn't be in that cheapy poster frame. Original graphite drawings (or portraits) can range from 25.00 up to 250.00, more if it from a name artist/illustrator.
