You want to create a poster out of a photo? There are two possible solutions:
PosteRazor, The Rasterbator or Posteriza offer a simple solution to create a poster using your A4 printer.
Load your photo and the software will cut it in several parts.
Print and assemble your poster.
As for Google Picasa: Go to the Creation menu > Create poster.
If you do not have a printer or simply do not want to spend the time editing your picutes, several online tools exist.
https://www.photowebr/- Between 1 to 30€ depending on the size and the desired number of copies).
https://www.smartphotor/- As from 29 €.
To get a good quality poster you require a minimum resolution of 300 pixels per inch). The larger poster, the higher the resolution of the source image must be.