Measuring 30x30cm, 12x12" 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 12x12", you'll need at least 864 x 864px and ideally 3600 x 3600px.
If you are looking for a frame with a mat for an 12x12" photo, choose the 16x16" (40x40cm) frame. The mat will have an 12x12" (30x30cm) opening.