Open CuteSoft_Client\CuteEditor\Dialogs\InsertImage.aspx.

 

In line 243, you will find the following code:

 

        try
        {
            img = System.Drawing.Image.FromStream(new System.IO.MemoryStream(data));
        }
        catch
        {
            return "Image is in the wrong format.";
        }

 

 

The logic is simple: we will show this error when the process of creating an Image from the specified data stream failed.