Yes, you can paint directly over existing wallpaper. However, there are a couple of concerns. The wallpaper seams will be more noticeable after applying the paint because the wallpaper pattern normally hides them. The only possible way to hide them is if you sponge-paint. Whether or not you use the sponge or the roller, it is important to apply a coat of primer so that the final coat will not soak into the wallpaper. If you want the perfectly flat wall, you will have to remove the wallpaper before painting. If the wallpaper was applied in the last few years, it may be dry strippable wallpaper. To find out, simply peel the wallpaper from a corner. If it peels with ease, then continue. If it does not, then you have the old-style wallpaper and a little bit of work will be required to remove the wallpaper completely. Rent an industrial steamer that will soften the existing wallpaper or use wallpaper stripping chemicals that work like paint strippers. Apply a primer/sealer, as you would have if you had not removed the wallpaper, and then a final coat of paint.