When creating a document template and using the Header block, all images that are uploaded in this block will be aligned left when exported to Microsoft Word, regardless of the location of the image in your template.
This is a known issue with Word so we cannot change anything in eTapestry that would fix this. You have a couple options to proceed:
1. Generate the letters in a PDF document. A PDF will align the header image correctly. 2. You can change the alignment by adding a line with a space above and below the image. Then highlight the image and the two lines and set them as centered. It will generate in Word with the proper alignment. Here's a video to demonstrate that process.