The resolution for professional photos depends on the outlet of the work. If the pictures are meant for web use only, you can have images 2400 x 1600 px, and they will display properly on any device. If the aim is to have a printed image, then you need to multiply the printing size by 300 DPI - which is the industry standard. For example, to print an 8 x 10-inch photo, you need a digital file to have 2400 by 3000 px. However, large format prints require less PPI - for example, a billboard is not 300 DPI but usually around 30 DPI. You can discuss with your client from the beginning what size they'll need.