donovanh

+1 for linking files. This also makes especially sense for files which are masked by vector elements and which use adjustment layers.
 
It would be even better to create blank image placeholders which can be filled automatically by drag&drop of multiple files. 
 
Reason: When you drop an image file it is inserted at that position at its maximum size and I have to resize it.
When I first create some image placeholder, I can drag&drop the images and they will be resized automatically to fit the placeholder.

I've fiddled with it some. Since you figured out symbols, I'm assuming you've already found the constraints tab in the same spot. I'd drag it somewhere other than the same group that has the layers tab so you can work with both easily as you experiment.
 
Now create a new layer, and put a shape inside of that layer. Select the shape and click on the dotted line connecting the bottom edge of inner square to the bottom edge of the outer square in the constraints tab. Now if you select the bottom edge of the layer and change its size, you should see that the inner shape stays the same distance from the bottom of the layer. You will also note that its height does not change, at least as long as there is room for it in the layer. If, however, you change the left or right edge, the inner shape will move proportionally with the layer since you didn't select those edges.
 
Now select the rectangle again and select the dotted line running from top to bottom of the inner square. (Leave the bottom anchor selected too.) Now if you move the bottom or the top edge of the layer, you will notice that not only does the shape stay a constant distance from the bottom edge, but the shape also grows proportionally with the remainder of the space.
 
What I haven't yet figured out is what those two little icons in the lower right corner with the inward and outward facing arrows in the circles do. If someone else has figured that part out, I'd be grateful.