Hey Mega_Cabbage, it's nice to see that you're looking to improve your graphics. You've definitely had a good start, though there are areas that need improvement, of course.
But before I delve into the real C+C, I just want to say that I actually disagree with some of what Isabella posted earlier. For the most part their post is solid, as their analysis of your weak points is spot on, but I don't think bokeh would have been the best option for improvement of that first banner. From the looks of it, you were going for a somewhat rough/grungy banner theme (the font style and color scheme lead me to say this), so splatter was definitely the way to go. Sometimes, light effects such as bokeh seem slightly contradictory or out of place paired with a grunge banner (unless there's a lot of shadows), so you do have to be a bit careful with using them.
Now that I got that aside, I'm going to talk about your Nightwing banner. First off, the mono color scheme is awesome. I really like what you did with blue and black. The top image and background blend together so well that you wouldn't even be able to tell that they started off as separate pieces. (Well, I'm presuming that the top image and background are separate. Although, if you just used a stock image - then that's still good coloring, lol.)
So you're definitely already at a good start with blending via coloring. However, with a better grasp on lighting, you can make monocolor banners a lot more dynamic. (You know, since that can distract viewers from the lack of color variety.)
Some easy ways to add lighting: 1) using black and white gradients set to Soft Light or Overlay, and 2) brushing with black and white in areas that need to be darkened and lightened and setting those layers to Soft Light or Overlay afterwards.
Here's an example:
(with lighting and darkening control)
(without lighting and darkening control)
I took a random render I had saved and used both of the methods I described above.
Now, I know the first version looks a lot darker and much less colorful, but you can always fix this later on with coloring adjustments. If we were to work with the second version, all we would be able to achieve is a flat-looking banner.
That's all I've got for now, though if you want some advice for C4D placement (as I'm assuming that's what you've used) and/or text placement, I can address those things in a separate post/link you to some resources.
Hope this is helpful :-]