I too have noticed a drop off in photos submitted in the last few days. It's fairly important to determine what caused the drop off if we can. It might be something totally outside of our control, e.g. the situation in Syria, but might be something like a change in banner display, which we do have some control over. Did the drop happen in most countries? We don't have full control over all Wikimedia banners - we do have to share the space with others, but perhaps they might be willing to delay some of their displays if we ask nicely.
I do agree that changing the banner from time-to-time can help. I don't agree that a banner of ""Participate in the world's largest photo-contest and help Wikipedia," would help any. The imperative verb "Participate" is very weak, and is not natural to American English speakers. It suggests a high school home economics teacher telling us that we have to bring cookies to the annual bake-off. Something more active is definitely needed, something along the line (but not exactly) of a high school football coach saying "Go out there and kick some butt!" That would definitely get some attention as a banner, but not the exact type of attention we want. A teaser ad might work however, e.g. "About your photos on Wikipedia .... (smaller type) upload them to Wiki Loves Monuments"
"Snap" might be a verb we want to use. It gives an idea of the action that we want people to take (snap a snapshot), unlike "participate." So perhaps "Snap a historic site, Wiki Loves Monuments" It could wake people up. Or maybe "Snap a snapshot for Wikipedia". As a noun "It's a snap, Wiki Loves Monuments"
I don't think these ideas are good enough yet for an actual banner, but I think that folks should brain-storm this and come up with new ideas. Straight informative banners can sound bureaucratic or just boring. Different languages or dialects should be considered separately, as translations are really tricky, e.g. "It's a snap" might mean something entirely different in British English
Pete
User:Smallbones