We've just added a new capability to PicMonkey: drag to select layers. If you want to select multiple layers on your canvas quickly, you can drag a selection box over the layers you want.
Start your drag-to-select action either by clicking on the work space outside the canvas, or by clicking on the background layer of your canvas. Release the cursor when you've selected the layers you want. Use the Multiple Layers palette to adjust your selected layers, or apply effects, textures, or edits.
Panning across the work space
If you are zoomed in on your multilayer design and want to move (pan) to another section to select multiple layers, hold down the space bar, click on your canvas until your cursor turns into a hand, and then drag to move your canvas freely atop the work space. (If you use two-finger trackpad motions, you don't need to hold down the space bar.)
To freely move your canvas up and down, and left and right, you must first deselect "Auto-center zoom" on the Workspace palette (accessible from the settings cog icon on the bottom tool bar).