The best way is to create a separate Class Library project for them, and include them there as Resources in the Resourses.resx file in the usual way. If you make sure that the each of the resources is declared
public
using the "Access Modifier" drop down on the Resources page, you can then add a reference to the DLL in your other projects and access the stings, images, etc via the static Properties object of that namespace:
Image myImage = myPropertiesProjectName.Properties.Resources.TheNameOfTheImageIWant;