Perennials under pine trees
You could absolutely plant some perennials under your trees. Your picture shows that you have shade to partial sun which gives you some good growing conditions. The main things you need to know is that the tree roots are close to the surface and too much damage to the roots may harm the tree. It's easiest to plant smaller plants and let them grow around the roots. You should never cover the roots with soil but you can use 2-3 inches of a natural mulch. You already have a good start on mulch with the needles you have.
I think you will get a lot of good ideas from this publication: http://www.extension.umn.edu/garden/yard-garden/landscaping/planting-under-trees/doc/8237pdf.pdf