I usually place the reference images at the top or the side of the main document and its "drawing area" ( Not really that specified, just an area that I feel comfortable working with. ) and put them in their own group.
I would put images in with different size's and possibly cut out some parts If there's like some specific part I'm interested in. ( just with Rectangular Marquee Tool and click Layer Mask icon. Nothing fancy. )
This of course requires a bit bigger document size, but it works well.
One thing to consider is the placement of reference images. If you need to make the document bigger for your drawing, you might not want to start moving the reference images around.
Basically what you can do is just select bunch of images from a folder and drag and drop into the open photoshop document.
Oh and of course you can copy image from the internets and just paste it into photoshop as well.