I believe they use Bing and Google aerial maps. Check to see the address is correctly listed on the zillow listing so it will correctly link to the appropriate aerial map.
The zestimate sometimes gives you a possible mortgage or rental per month payment. Most of the time this occurs when the property is not currently for sale and the default is the rental/mortgage payment amount
www.postlets.com is a free way to get it on Zillow and several other sites
Yes the best way to find market value for your home is to have a licensed agent do a Competitve Market analysis which analyzes what has sold in the area in the last several months. With the market continuously changing, values can vary month to month based on recent sales.
You should be able to pull the tax record on your home from http://www.dat.state.md.us/sdatweb/taxassess.htmlYou should be able to pull general info on the house up on www.zillow.com as well by entering the address/zip in the search fields
Make sure to hit save at the bottom after you add the photos or they won't upload
I think a patio with some steps from the house would add excellent curb appeal and value so you can enjoy your yard and extend your living to the outdoors.
You can edit the listing if you are registered on zillow as the owner of the property and you should be able to correct the details.
Hire an appraiser to evaluate the property and find out its true market value.
If you are interested in a comparative market anaylsis of this home in Gwynn Oak, please contact me and I would be happy to email you one.