Well .. how can you be sure that this is not a problem with support for Windows BMP format in your GIMP? Please trying Paint on Windows or any image processing program (from Irfanview to Photoshop!)? If you are a Linux users therealso are a lot of small image viewers available. Also the complexity of the canvas should not be a problem. This is a 'pixel-to-pixel' copy operation.
Please see attached file. It works for me on at least 10 different image-editors and -viewers. Could you try this file? And maybe you also could share a file? If you use SQLyog on Linux/Wine (what it looks like) please tell the exact Linux and Wine version as well as what destop (including version) you use (Gnome., KDE, Uniity or whatever).
But please provide requested environment details and a sample file and we will check what we can do.