Were you able to upload images before sunday?
If you’re using Linux as I am, I had a problem like that in the beginning to. I’ve fixed it by giving write-acces on the folders in wp-content/uploads/ and I even had to make all the folders manually myself (make a map ‘2012’ and put in that folder all the months, so a folder ’01’, ’02’, … and give write-acces to all those folders too…
Working like a charm ever since.