You don't provide enough details, maybe you could keep one unique UVmap and mix your pictures with a MixRGB node (as explained further), but it might simplify your work if you create 2 different UVmaps:

Now create 1 picture for each UVmap, for the second one keep a transparent background (when you create your picture, set RGB and Alpha values to 0):

In the Node (Shader) Editor create 2 Input > UV Map nodes and select the 2 different UVmaps you've created, plug them into a Color > MixRGB node (by default choose the Mix mode), plug the 2nd Image Texture's Alpha output into the MixRGB factor. Then MixRGB > Diffuse > Material Output. If you only need one UVmap, skip the creation of the 2 UV Map nodes.

Now your background and your pictures mix on the surface of your object:

You don't provide enough details, maybe you could keep one unique UVmap and mix your pictures with a MixRGB node (as explained further), but it might simplify your work if you create 2 different UVmaps:

Now create 1 picture for each UVmap, for the second one keep a transparent background (when you create your picture, set RGB and Alpha values to 0). You can also export the UVmap and work the pictures in an image editor like Photoshop or Krita:

In the Node (Shader) Editor create 2 Input > UV Map nodes and select the 2 different UVmaps you've created, plug them into a Color > MixRGB node (by default choose the Mix mode), plug the 2nd Image Texture's Alpha output into the MixRGB factor. Then MixRGB > Diffuse > Material Output. If you only need one UVmap, skip the creation of the 2 UV Map nodes.

Now your background and your pictures mix on the surface of your object:

You don't provide enough details, maybe you could keep one unique UVmap and mix your pictures with a MixRGB node (as explained further), but it might simplify your work if you create 2 different UVmaps:

Now create 1 picture for each UVmap, for the second one keep a transparent background (when you create your picture, set RGB and Alpha values at 0):

In the Node (Shader) Editor create 2 Input > UV Map nodes and select the 2 different UVmaps you've created, plug them into a Color > MixRGB node (by default choose the Mix mode), plug the 2nd Image Texture's Alpha output into the MixRGB factor. Then MixRGB > Diffuse > Material Output. If you only need one UVmap, skip the creation of the 2 UV Map nodes.

Now your background and your pictures mix on the surface of your object:

You don't provide enough details, maybe you could keep one unique UVmap and mix your pictures with a MixRGB node (as explained further), but it might simplify your work if you create 2 different UVmaps:

Now create 1 picture for each UVmap, for the second one keep a transparent background (when you create your picture, set RGB and Alpha values to 0):

In the Node (Shader) Editor create 2 Input > UV Map nodes and select the 2 different UVmaps you've created, plug them into a Color > MixRGB node (by default choose the Mix mode), plug the 2nd Image Texture's Alpha output into the MixRGB factor. Then MixRGB > Diffuse > Material Output. If you only need one UVmap, skip the creation of the 2 UV Map nodes.

Now your background and your pictures mix on the surface of your object: