Is this plagiarizing?

Not really, if you didnt see the copyright symbol on the picture then no.

Yes. Totally.

The original image belongs to the original artist whether there is a copyright symbol there or not. You've stolen that person's work, which will disqualify you from the competition.

Get a camera and take your own photos to use in your art. Many artists I know do this, and they would so totally NOT base a piece on another's image.

It's best to print out a picture, tape it to the corner of your easel/paper, then use it as inspiration, and paint your own. Using someone else imagine in a collaboration of sorts is copyright volition. Either make it your own, or paint one yourself.

Not really I mean, if it's really unique, like a tiger wearing clothes or doing a cartwheel, it might be recognized, but if it's just a tiger walking around, loads of people have done that before it's alright

How did you incorporate the pictures into your painting?

If you traced them, it is possibly plagiarism. If you just looked at them for ideas, it isn't.

If you're using them as references then no. If you're tracing, yes.

You could always credit the source if you're worried.