Note that the document is hundreds of pages long, and contains text, images, tables, etc.
You don’t give information about your system or the structure of your evidently long and complicated document. So I will assume Windows 7/LO 6.0 (or something similar) and a document with and unknown number of page styles.
Try making a copy of your document (for safety’s sake). Then insert a new first page to contain your image
-
Put the cursor at the beginning of the document. Open the Styles and Formatting list. Make a note of the page style used (very likely Default Style).
-
Adjust the First Page style available in the styles list to suit your document by right-clicking on First Page in the list and selecting Modify/Organiser. Change Next Style to whatever style you noted for the beginning of your original document.
-
Create a new page by putting the cursor at the beginning of the document and selecting Insert/Manual Break and set its style to First Page.
You should now have a first page to contain your image. You can modify its layout (eg to mmake it suitable for a cover) if you want without upsetting the rest of your document.
Step 2, of course, probably won’t be necessary if your document uses Default Style but it never does any harm to check.
An afterthought
If your document is intended for double sided printing with mirrored pages the new page might upset the position of page numbers (if you have them). But that is easily solved and I think has been dealt with in earlier questions.