Creating a poster - need help with upscaling

[EricT]

Hello,

I'm making a poster containing some skateboard pictures of a friend of mine's kid.
The poster will look something like this:

I want the small pictures to be about 4x6 inches, which means that the background picture should be about 18x10 inches (16:9 format). Currently the picture has a resolution of 3896x2192 pixels. Now my question is, should I downsize the smaller images and then send it to the lab and let them upsize the whole poster, or should I upscale the background image myself? To have 300dpi, I would need to upscale to about 5450x3066 (some rough math here, but that's beside the point).

I'm leaning towards the latter option, because that would give me the best resolution on the 3 smaller pictures. Any advice is appreciated, I would also like input on the poster itself!

[Wheatfield]

Start with a canvas the size of the poster.
Size and paste the background on and adjust it to make it look right.
Past the other pictures on and adjust each of them to look right.
I'm not sure if the library look suits this, I think I'd be moving the pictures around a bit, and perhaps turning them diagonally.

[David Glazebrook]

If your printing on a wide format poster printer your resolution can drop as low as 150dpi with degrading the image quality. If your print though on a photo printer this a different story as it will require atleast 240dpi.