Based on the information you have provided, the only response I can think of is:

Did you change the image's filename from it's original extension to the bitmap (.bmp) extension using the "rename" option, or did you actually use an image editor to "save as" the bitmap image file type?

If you just renamed the file, this could be your problem, you should do the latter, as just changing the file extension does not properly convert the file from one format to another.

If this is not the case, then perhaps the file is corrupt or damaged in some way.

Otherwise, to ensure you have inserted this file correctly using Designer, the proper method is to:
1. Click on the Designer's Insert Bitmap Image button (the little globe icon, 3rd from the right).
2. Click on the approximate position on the invoice spec where you want to insert the .bmp image.
3. A dialogue box will come up. Find and Select the .bmp file from the filenames that come up, then click OK.

If the image is not the size you want, then you should edit it with an image editor, save it as a new filename (to retain the original's size format), then try the procedure again, as Invoices Designer is not an image editor and the image may not print the way you would like if resizing it in Designer.

Hope this helps.