For those who are still having this problem, here is what worked for me (plus a test post to see if it actually works...)
I couldn't get a large picture to show up using the actual "Signature Picture" functions (URL or upload) on the "edit your signature" page, but if you have the photo on a photo service or another website, you can just enter the URL using the "Insert Image" function in the text/msg edit box itself, and that made mine showed up the correct size (despite being the same URL I tried in the signature pic URL function). Good luck.