Facebook Image Preview

Why does the TreeMac logo on the following page www.treemac.com not register as a preview image on FaceBook when Sharing? Last time I shared the page it worked but more recently it doesn't, the only thing I changed was giving it a clear background. The size is pretty much the same as before. I have tried adding rel="image_src" as I read in another stackoverflow thread that this could help but it hasn't. Any ideas?