I need some tips on what else I can do with this before uploading. I am aware that the extraction of the scan is a bit poor, I'll fix it when I get home. So, any helpful ideas are welcome :)

Well for me the scan is a bit blurry but as the background also does it's a pretty good thing cause it makes the entire wall pretty much all in the same style and gives a nice feeling of strange light night...

It's a very good wall but for me you also need to add some clouds (smaller ones) on the right part of the sky... Around the building... Just to even a bit more the top part of the sky... But of course you don't have to.

Why not put the character a bit more on the right? Just under the moon? It would balance the composition...

Don't forget you need to correct the extraction indeed especially on the right bottom side of it and you HAVE to add your sign on this wall.

Good job so far! :)

Great work! love what you've done with it, and it shows loads of potential :)

as zazou said, the scan's kinda blurry, but it gives the wall a good characteristic in this case. the colours are great, and don't forget to add your siggy there XD

keep the great work up!