I have no experience with macs, but you should be able to copy the original picture, crop the copy, and import the copy here though. To do that in a thread you'll have to make sure a cropped picture is saved in your computer, reply to whatever thread you want to post it in, click 'go advanced' at the bottom of the text box, then click attachments when you get to the bigger text box with more options. Then click the 'add' button to manually add pictures from your computer to LF. Once you do that you can attach them to responses in threads. Using a picture for an avatar should be much easier. Keep in mind though that these are the steps that I personally take, so using a mac may be slightly (or very) different.