I have a large oil painting signed by an artist named Dickinson. Is this worth anything?

An art pieces value all depends on how well it's described. If you just say it's got some birds, rocks, and a boat will you buy it? No one is going to say yes. You need to describe the art from the perspective of the artist. Does the art piece symbolize anything. Is there a deeper meaning than just your initial perception? These are all important questions you need to ask yourself before you can describe the piece in a way you can sell it for a high value. Even large copied oil paintings cost over $150 but if you feel like it's worth more than ask for more and describe the painting better.

Try using the website http://www.whatsitworthtoyou.com. For ten dollars they'll appraise things for you from a photo. Fast, easy, cheap. You can get a list of their experts from their website with their qualifications.
I'd send several views of the painting and a close up of the signature. It's not a perfect appraisal system since technically the best way to appraise something is to be able to touch it and inspect it as well as research it, but it's a good and inexpensive starting point. Also there is a web site where you can submit digital pictures of your item, pay about $6.50 and receive a report with a price estimate within 48 hours. It is staffed by 19 experts, all of whom have worked for either Sotheby's and Christie's auction houses. www.valuemystuffnow.com I just haven't used them in the past.

For certified appriasers go to: http://www.princeton.edu/~rbsc/department/graphicarts/appraisers.pdf

Try: http://www.christies.com/valuationdays/ This gives locations and who to contact, as well as email and phone numbers for Christies Valuation Days. Christie's Specialists travel the world in search of new and exciting treasures. Browse the itineraries below to see if one is visiting a location near you, and schedule an appointment to discuss the sale of your property. If you do not see a particular location or specialist category listed or have a general consignment inquiry, please visit Auction Estimates or email info@christies.com.

Also this site has a new signature section: http://www.findartinfo.com/ but it may cost you to use it. Same with their artist section.