[Doitsu]

@DiapDealer:
I wasn't able to reproduce the OP's issue either, but found out that Sigil doesn't properly resolve stylesheet image references if an epub contains two images with the same file name in different subfolders.

For example, my (valid) test epub contains two different versions of blue.jpg in different subfolders. After opening with Sigil, the second version of blue.jpg was renamed to blue0001.jpg, but the CSS file reference wasn't updated.

Before:

#blue_one {
background-image: url('../images/blueone/blue.jpg');
}

#blue_two {
background-image: url('../images/bluetwo/blue.jpg');
}

After:

#blue_one {
background-image: url('../Images/blue.jpg');
}

#blue_two {
background-image: url('../Images/blue.jpg');
}

I realize that the end user is highly unlikely to encounter such a poorly designed epub in real life, but, IMHO, it couldn't hurt to check for this.