How can I lock a background image, so that it becomes non-selectable ?

Hi, I was having the same issue and figured out a simple work around that was useful for me. With my document open I selected the Layers tab on the left side of the screen. I used the Options drop down menu from there to select Import as Layer... I sselected the image I wanted as my background image and near the bottom of the dialouge box I selected Order: Appear behind page. Then when I need to select text areas that are on top of the background image I use the Layers tab to hide the background image layer I created by clicking the little eye symbol. This has worked for me so far.

Please explain more about your issue.  Selectable by what tool? in what type and version of Acrobat? Can you provide a screenshot?

But, just in general, everything viewable in Acrobat Professional is editable. However, if a document is signed with a digital certificate or has special Reader extensions applied, then Acrobat behaves nicely and blocks editing. But this is a courtesy, other viewers and tools can still modify the document, although they'll break the certification in the process. So with a signed(certified) PDF you can make Acrobat and other compliant viewers block editing, and if the PDF is modified by another tool you can detect the changes, but you can't completely protect content from copying and manipulation.