Hi there, The WG are most likely the initials of the company that made the item, the designer or the location of where it was made. Without seeing the other markings it's hard to tell. Is most likely from the UK based on the layout of symbols/markings. The image of certain animals can tell you a lot, such as a lion, etc. Based on the appearance and engraving on the front, I would say it is most likely from the early 1900s and would be described as "Art Nouveau" when you go to sell it.
Lockets from this era in sterling silver are selling for around $30 (USD) to $100 (USD) depending on the condition, quality of the engraving and weight of the whole item - this is without knowing the designer. If it is a desirable, well known designer it could possibly go up from there in price. Check out antique UK hallmarks. There are a lot of excellent websites specifically for UK marks.
Whenever I can't see the markings well, I've found that taking high resolution photos of the marks helps me see them better as I can view them up close on my computer.
