How can I have more control over where pictures are placed?

If Collage.com's automatic layout is limiting the design you have in mind, try unlocking the layout

Free Form Mode / Unlock layout

To turn the automated system off, click the  or  button to the left of the project (above an individual page in book/calendar project/below the project on mobile).

Then click the Free-form option (Unlock Layout on some views). 

On some projects, activating free form mode makes the safety zone visible, giving you access to the full printing area.

Anything within the safety zone will be on the design. Anything outside it, may be cut at printing. If you want your design to cover the entire product, the entire red cut area must be covered by the image even though that will be cut off.  

You can also activate the Snap To Grid mode that will appear below the Free-form button once you're working from this mode. The snap to grid mode still allows complete control over photo placement but offers the added benefit of automatically snapping to a grid line so that you can align the photos evenly. 

Overlap Options

Free Form Mode allows you to stack images on top of each other in your design. In order to bring an image on top of another image, or push an image behind another overlapping image, click the photo, then click   to bring the image forward and    to push the image back.

If you decide you want to turn the automated layout back on, click Automatic or Exit Freeform Mode.  All free form work will be undone if you switch off Freeform mode.