FilkoSE, the log files are indicating that its failing on trying to create the preview images from your screenshot. I see in your screenshot name it appears to be a BMP instead of a JPG. Make sure you are uploading a .jpg file.
Fairyy, the last item I see you try to upload was your screenshot Enchanted Garden-DX Theme and that appears to have come across just fine. So I'm not seeing any recent failed uploads from your account.