The first thing you need is a picture that you like. It can be any size you like (within reason). The best place to find images is google.. just search for whatever you want a picture of an then click 'images' at the top.
You can right click on the image and select 'save as' to save it to your desktop. Here's me searching for a picture of Bilbo..
Once you have your picture saved to your desktop (or any other folder that you can remember) go to Field to Farm forum and login.
Click your name as shown in the picture below to get the dropdown box appear. Then click the link 'My Profile'.
In the 'my profile' page you should see a greyed out person as per the picture below. You'll notice that in the top left hand corner of the greyed out person there is a link that says 'change'. You can't see it very well in the picture below but it's definitely there, right under the word Bilbo in the green box.
When you click 'change' in the picture box above you should be asked to upload a photo. Click the 'Use a custom photo' circle button below and then click 'choose file'. Another box will popup asking what file you would like to use. Choose the picture that you saved to your desktop earlier (or any other photo you wish to use).
When happy you can click Done and you should have a profile picture when using the forum in future.