When you read a post or view a picture that you want to thumb, look at the bottom right corner of that post. There is a thumbs-up there -just click it to give one.
You can see how many acorns you have on your profile page http://garden.org/users/profil...
. You can give from there, or you can click the acorn icon in a post/picture you've enjoyed to send one/some that way.