First analyze the concept of a 'bicycle.' It is actually a frame & fork with a collection of components that are supposed to work in harmony with the frame & fork. Now this bicycle has been sitting around for 5 years. And you're having difficulty with the rear-derailleur (RD) not shifting. And I'm not surprised.

During long-term storage, the grease and oil and dirt and grime can form something that works best as a glue - not a lubricant. So I would suggest a thorough cleaning of both front and rear derailleurs. WD-40 will work well for dislodging grime - just don't use it as a lube. As the cable-housing has likely become internally affected by the grime - get new housing. And new cables as well. The brake-cables look good from here, but I'd replace them as well - to be safe. Once both derailleurs are clean, add one drop of good oil to every moving part - bolts, pivots, springs, etc. Then replace the cables and housing. Here are instructions for doing the adjustment on these components:

http://www.parktool.com/repair/readhowto.asp?id=64

http://www.parktool.com/repair/readhowto.asp?id=75

http://bicycletutor.com/adjust-rear-derailleur/

http://bicycletutor.com/adjust-front-derailer/

Or: Bring it to a shop you trust and ask them to do this for you.

Your old bike looks to be in fine condition. Get it fixed-up and ride it before you think of selling it. Looks like a nice bike!