here are the answers to your questions Am i responsible if one of my users posts and image without the owners consent?
Yes you are very much responsible for this as the content will be lying on your web-servers, which you are required to remove when you recieve notification about it from the original owner or anyone else for that matter.
During user registration are there certain conditions that i could require to protect me legally if one of my users violates copyright laws?
Yes you can put terms & conditions or terms of use in words for users to read and accept that they will be solely responsible for their actions on the site. Plus they (registered users) will not indulge in uploading copyrighted material on the site. if they do, you will be forced to remove the material in question right away (and without any notice).
Is there any website that i can go to that puts this in perspective that i can understand without all the confusing legal terms?
yes, you can visit youtube.com and check out these two links for best available information: www.youtube.com/t/terms & www.youtube.com/t/copyright_center
If you are looking for information related to images only, you can goto flickr.com and check out this section: www.info.yahoo.com/legal/us/yahoo/copyright/en-us/
Let me know if you need more help. I'm going to be in need of similar checks once I am done with working on www.partyImages.com