Something I quickly cooked up for C3. Basically, a somewhat stylized background with plenty of mountains located further away, and some sort of lake/river/pond located nearer. A screenshot showing clearly how the background is intended to look like can be found here.
Included in the ZIP file are two ExGFX files (in which ExGFX86.bin is supposed to be inserted into the BG1 slot, and ExGFX87.bin, into the FG2 slot), a PAL file (palette), a Map16Page.bin file (for obvious reasons) and a sample level (MWL file), showing how the background is supposed to be built up. Credit is not necessary, but would be appreciated.
Small note: I recommend setting the horizontal BG scroll in the level where you plan on using this BG to "none", otherwise the background would look a tad too repetitive.
On another note, please redownload this if you had downloaded the version I had submitted before - I managed to find a small cut-off tile in the background, and thus I had to do a few adjustements and resubmit it.