When attaching an image in Microstation V8i there is an attachment settings dialog that comes up after selecting the image to attached and selecting open. This dialog has many settings for the raster image. You can change these defaults by going to workspace - preferences - raster manager - default attributes tab and then click on the "Set Default Attributes" button. This will open up the same Attachment Settings dialog as when you attach an image. Set the defaults to the desired values and click OK to exit the dialog and then click OK to close the preferences dialog. Then in Micrsostation do a file - save settings. These settings are stored in the workspace's user preferences file (.upf). If this is ever deleted or a different workspace is used these defaults will need to be set again.