I'll preface this by saying that I paint my flags by hand, on thin metal, laminated in layers. So, this is an educated guess, but I would think that regular bond paper, such as you probably use in your inkjet or laser printer, would be strong enough, especially if you wind up with a 2-layer lamination, with white glue as the adhesive.
So, if your image shows the obverse and reverse of a flag, joined at the staff sleeve, so that you would fold it around your flag staff, I would coat the blank side with a thin layer of white glue and fold it around the staff. Before it sets, or while it sets, you could shape some gentle curves in it.
If the image consists of 2 separate pieces for the obverse and reverse, the process is almost the same, that is, to glue it together, but you'd need to add the piece to serve as the sleeve around the staff.
Like I said, that is an educated guess, from working with materials like that, but I've never tried it, because I use metal.
Hope that helps, and I'm curious to read what the other painters/modelers' suggestions might be.
Prosit!
Bradley