Phoster is an easy way to make stylish simple posters, quite useful when you want to advertise events in school.
You have a wide range of poster templates to choose from, all you need to do it add in an image (you only get to do 1 image, so choose wisely!) and tweak the text. Then you can save it to the integrated gallery, or export it to your camera roll.
Here is one of the posters I made, to advertise the YouTube channel.
I like it because it is simple, effective (and only took me about 5 mins to make).