What worked for me last time I needed something bigger then A4 is asking a shop that does wedding photos, birth/funeral/business cards and the like. They had an impressive A2 printer that did the job. Wasn't expensive either. There is no DPI problem. Just get the highest resolution picture you can ...

Great thread guys as you can see I'm new!love the dig , loom ,monkey island ect....Dont mean to sound dumb... if ive got this right? what you do is take the images and find companys that will produce them into real posters is that right? From what ive gathered the images have to have a certain value...