A good tip is, if you are using screencaps, try to use images from the same scene, or that have a similar background or tone to them - this makes it easier to blend in when you are first starting out, as you can use the backgorund of them to blend the images into one another - or if you are using photoshoot images, try and get ones from the same shoot as well.
Once you become accompished at blending, you can then use images from different things, as you will find it alot easier to blend the images together.
Also, a tip on blending - if you can zoom in and reduce your brush size, you can get right up to the edge of the image, if you need to erase background. Some people prefer masking, but I tend to mask only so far, and then manually use a brush to get rid of the last bit.......
Even the most simplest of banners can look stylish if there is flawless blending! And of course, practice, practice practice 'lol' 🙂