If you planning on selling the app then its best to ask the site but if doing the app for free, then personally I would ask the site still but it won't matter as much
Check their copyright policy/statement. Many will allow you to use the pics as you wish for your own personal pleasure, but if you plan to profit from their use, they'll usually need to be contact and permission obtained. Better to clear the air in the beginning than face a copyright challenge to an app that's become successful and so much time and work have gone into putting that app together.
If you've never used java or coding at all, yeah, it's pretty tough jumping into things. I've done some html in the past, so I'm understanding a little, but it takes some time and help to really fully understand what you are doing. I'm all about trial and error, haha.