insert images into background/posts

ok hello all…
i have read high and low and tried multiple plugins without success..what i am trying to achieve is…to have a different background image when you click on a category. ie…category being lemons i want the background image of page to be lemons…the inoplugin only works if you buy the premium version. i tried that one and sent creator an email…boo hoo…
the other thing trying to accomplish is one or the other of which is to have the post have a background image or have a thumbnail of that post’s category appear in post but not just when you go into the post…so to explain when i go to a category that all the posts listed have their little thumbnail showing…i am using the sliding doors theme by wayne conner. again i have tried multiple plugins..i am looking for some help in what i might be doing wrong…..oh i would like to automatically assign a thumbnail to post according to category assuming i can figure out a way to get them to show up…and even in using set featured image in the post nothing happens..ok well site is onehundredclicks.co.cc and you can email me at [email moderated per forum rules] …thanks to all in advanced and i hope i explained well enough…ps i know nothing about code