The first thing you need to do is get your negatives scanned. If you want to get large format prints done, you are best getting them professionally scanned to a high resolution. Once you have digital files, they can be manipulated in photoshop (other editing programs will do what you want too) to get the effect you want.
Were the photos shot in colour, or on black & white film? If they were shot in black and white, all you can do is apply a tint to the area you want. If they were shot in colour, then you can leave the bouquet in colour and make the rest of the image black and white.
A photography studio can do this for you, and they will also ensure that the photo is at the best quality for the reproduction you want. So whilst it may be a more expensive option, it may yield better results. Also talk to the printer you intend to use, they may offer in-house design services.
Bookmarks