Well, until such a feature has been implemented into Writer, I think you could temporarily solve your issue by using a second program and then inserting them into writer.
A4 paper is 11.7 x 8.3 in so if you use a program like Photoshop, or GIMP(its free counterpart) you can scale several images at once. If you’d prefer to download neither, you can use a website like Pic Resize(url: http://www.picresize.com/batch.php )as well. If you do this regularly and in massive quantities, I would recommend getting a program you can run locally as this will prove to be the more efficient method.
After you’ve changed all of the dimensions of your image to match that of the A4 paper, you can, in explorer, highlight them all and drag them onto your document where they will, hopefully, be the size you wish them to be. However, I believe you’ll have to manually move them all to the background because I don’t think they’ll automatically be moved there.
Try this method with two or three pictures before you move onto large batches though. I haven’t tested it myself but, in theory, it should work.