I am trying to make a skin for the Air-Cooled-Cup mod.
The problem is that the file won't load in LFS.
What I have done:
1. Created a template in LFS.
2. Drawn on top of it in Adobe Illustrator.
3. Exported it as JPEG with the name "4BD339_M5" into LFS/data/skins

Can anybody help me?

I downloaded it and indeed doesn't load in LFS. Not sure if Illustrator being a vector graphics editor have something to do with it, which I doubt, but you never know ¯\_(ツ)_/¯

But then I copied the image instead of saving it, pasted it in paint.net, saved as jpg and it worked. Maybe try using a different image editor (preferably a raster editor e.g. paint.net, Photoshop, GIMP, etc etc) and see if it works?

JPG only! NOT JPEG