Don't worry about it! Everyone does it the first time they post something.
You can take a screenshot in numerous ways, but I'll give you one solid method:
1 - Start JKA
2 - Open the console and type
devmap mp/ffa3
3 - Once the game loads, put on the skin you're going to take a picture of.
4 - Go into a bright area on the map.
5 - Open the console and type
cg_thirdpersonAngle 180,
cg_thirdpersonVertOffset -20, and finally
cg_thirdpersonRange 50
6 - Now, in the console type
bind p screenshot
7 - Close the console, then
press p. This will save an image of your current screen into either your JAplus base folder or your JKA standard base folder.
8 - Check both of those locations for a folder named "screenshots" . In there, you'll find the image you just took. You can then upload it by using any file-sharing website. An example would be
http://tinypic.com/.
I hope this helps!