That tool/mode where you have a background image and you can start modeling things, create meshes and etc. using that image as reference? Also: Can ANY image be used in this mode/tool?
Note: This is for Blender 2.79
I think the mode you mean would be
Edit Mode. To insert a background image, press N then scroll down to
Background Image. Expand the panel then click
Add Image. Click
Open and select your image file. Make sure the checkbox next to Background Image is ticked! And you can use any image, but make sure it is a compatible file format and that you have the rights to use the image.