How to Place A Photo On The Desired Layer

It sound like you have a template with a smart object layer that has some design and you want to change the design. There are two ways you can doe this.  First you need to know that all Smart Object layers have an associated object transform which may include distortion and warping.  The transform will not be changed when you change the Design.  Therefore your changed design must have the same aspect ratio, size and resolution as the the smart object in the template.  So you can use menu Layer>Smart Objects>Replace Content... to replace the smart object with a different image  design.  Or you can double click on the layers content thumbnail in the layer palette to open the object for editing.  The layer's Object will open in ACR, Or Illustrator or Photoshop. Do not changes the size of the object.  When you commit the changes Photoshop will replace the smart object layer with you updated object 

Adding a smart object will always create a new layer. Pasting will also create a new layer. What you need to do is create the new layer and merge down to combine it with the layer that you want it pasted into, or use blend modes to combine the layers.