Bug on image processing

[fotisps]

I have a bug when I am uploading an image and I have tested it in two different themes, so I think it might be a bug of themes but it may be an issue on flags when the thumbnails are being generated. When I am uploading an image, the image is being uploaded but on the background I get a session timeout. Sometimes is visible, sometimes is not. So when I press "update", nothing is updated but the image is there. Other time I get this error message: "Post-processing of the image failed likely because the server is busy or does not have enough resources. Uploading a smaller image may help. Suggested maximum size is 2500 pixels." The image is small (1000x1000) and my server is not busy at all.