classifieds adding pixs for my sale ad

I dont know what the deal is but I never had problems before adding my pixs now it wont let me saying something about only png gif jpg jpeg ….what the heck is that ! ?
I dont understand it and I am not that computer savvy to get it.
so my sale ad for possible sale cavern spring baby kirin #2 is up but i couldnt add pixs. So if anyone wants to see ill have to email them to you. And if someone can explain how to load the pixs in a way ill understand ill see if I can add them.