Im having this problem too - very frustrating! Sunday is my main listing day and I have tonnes to list this afternoon and tonight - any images i try to upload have failed and the ones I have already posted to ebay have vanished - I dont know whether to cry or have a huge hernia!!!!
Im getting pretty fed up with the unreliability of Auctivas image hosting services of late - a few weeks ago I resized all my images so they would comply and now it is failing again.
Please sort this out soon Auctiva - your British users are losing alot of time and money over these failures - Sunday is a prime listing day!