You need to have been a member for 3 months and have a 100 posts, or for 6 months and have 50 posts (I think).
When you take pictures, you can either take them in the smallest size, or crop them afterwards.

Go to the classifieds homepage
click the section where you want your add to be in
There's a small button in the top right hand corner saying "place an ad"
Just fill out the information. One thing I made a mistake on once is that the "Buy it now" price needs to be the same as the asking price. The website will automatically add the shipping costs you stated them to be.