There is no agreed upon standard definition for the term "photo safe". While there isn't a set standard or standard tests to follow to determine if a photographic article is stable and safe from degrading over time, there is a coordinated and concerted effort in the archival community to further develope and distribute science-based preservation information and guidelines towards ensuring photo longevity.
JournalsandBooks.com's definition is well suited for scrapbooks. It is:
Using materials and methods that tend to prolong the life and quality of photographs for as long as practical.
The easiest steps to preserve photos are:
Here is an exerpt from a more tecnical answer that is a result of an industry-wide movement to develope standards:
"...the SPS's definition of photo-safe: Although a photographic activity test exists, it takes into account only some of the possible chemical interactions that can occur with photographic materials. There is currently no standard definition or set of test methods that fully encompasses the term photo-safe. The term is often used in advertising to imply a product's non- reactivity with photographic images; however, the basis for such a claim is unclear. So why is the SPS definition for lignin-free better? Because it is quantifiable (you can give it a number) and verifiable (someone else can check the result)..."