Measuring 30x40cm, 12x16" is a common size for photo prints, art work and posters. The standard way to measure a print is by measuring width by height.
To get the best print result for 12x16", you'll need at least 864 x 1152px and ideally 3600 x 4800px.
If you are looking for a frame with a mat for an 12x16" photo, choose the 16x20" (40x48cm) frame. The mat will have an 12x16" (30x40cm) opening.