Level designer tool

Hi everyone,

Can anyone tell me what can be the best tool for making/designing levels for a 2D platformer to be made for mobile? I am aiming for a free tool which can help me make all the background for the levels I need.

Please suggest the best tool for this. It would be great if someone can also provide links for free backgrounds which can be used in my game.

With regards,

Shahjada Md. Tanvir Ahmed