Just an artistic suggestion.....
Taking a bunch of characters from promo artwork and slapping them on a marquee or cp has been done. Not to mention that it's tacky unless you really have a good feel for color matching and design. Don't get mad, but those two designs you showed kind of clash due to the fact that there are just too many diverse characters on there.
But I didn't come here to critique I came to give advice. Here are the main things you should be doing in no particular order or importance.
1. Pick a theme. Every game you've ever liked is NOT a theme. Nor is "the classics" or "street fighters" that's just too many characters whith drastically different color schemes, drawing styles and designs. Now something like "street thugs" (characters from double dragon, bad dudes, ect...) or "pac's" or "classic insert company name here" would be. Why? Because if it's by the same company in the same time period then it's likely to have the same artists' work. If it's the same type of character (eg street thugs or pac's) then they will look similar and it will be much easier to make them match up.
2. Pick a color scheme and stick to it. That doesn't mean everything blue (it can be though). That means pick a few colors and make sure each item has at least one of those exact colors in it, preferably two or more. If all or your character art is made by the same artist then this isn't as much of an issue as long as you balance the characters with the same colors throughout your surface, but if they aren't then it's crucial. If the characters don't match up then don't panic. That's what the replace color function in photoshop is for. What? You don't have photoshop? Well you better get it or something similar as paint shop pro just won't cut it if you want to do a good job. You are going to need high-end color adjustments and/or professional quality filters to make dissimilar artwork mesh together.
3. Unless your artwork has a LOT of pale blue in it slapping the default mame logo in the center, as-is is going to look awful. Don't panic though, it's easily fixed.... The easiest way to do it is to find the dominant color in your piece and use a color overlay filter to paint that color or a complimentary color over the mame logo. Just doing that, without doing anything else to your entire piece will make a WORLD of difference. Alternately, if you like the mame logo's color you can find an artistic filter that mimics the brush style of the artwork you put in the picture. That will make it mesh as well, but it's much harder to pull off without a professional eye or artistic talent.
4(a) Don't use a solid color background. Put a little bit of pattern or movement back there. If done. Properly then it'll make your artwork really pop off the page.
4(b) Don't make your background more complicated than your foreground. Your background should have much duller colors (lower the saturation) and possibly be out of focus a tad to show that it's in the back. It shoud also have little to no detail unless it is meant to be the background for said characters or you really know what you are doing.
5. As was previously said on this post... Less IS more. Narrowing it down to 20 or 30 is not less. Narrowing it down to 5 or 6 is. Again, you can do more, but you have to really know what you are doing. But just a hint.... If you think you do know what you are doing, you have 20 or 30 on one piece and there isn't a significant amount of layering/overlapping of the characters going on, then you probably don't know what you are doing and need to cut it down a tad. Putting dozens of characters with none of them touching makes for a very flat, tacky, layout. But then again having dozens of characters overlapped in a space that only about 5 or 6 should be in isn't any better. Again, if you aren't good at this stuff then keep it simple.
Ok now I hope that helps. The reason I took the time to do this is I've seen this character hodgepodge a lot and they look very unoriginal and generic despite the personality and hardwork the authors have put into them. If you use the steps above, your artwork will take on a personality all it's own and it will look great even with no artistic talent, what-so-ever. If you have talent then it's gonna turn out fan frikkin tastic.
Trust me on this, with just a little bit of work you can turn those bleh collages or other peoples' art into something original and unique to you.
I have faith in you guys.....
You can do it!
