Welcome to TVPaint Community forums !

Do I have to upgrade to professional , to be able to load a long or larger than 1980x1020 background , to be able to pan this background , as is required for a walk cycle ?

It's not mandatory. Professional Edition runs in 64 bit so it can use much more memory than the Standard Edition, which runs in 32 bits only.
Larger files = bigger data = bigger memory to manage it, but you can still use the Standard edition and give it a try.

You need to make your project as big as your complete background image. let's say 3840x1080, cause TVPaint cuts everything away what isn't inside your project dimensions. Than you animate in this size and export it to do the pan in some other software (like After Effects) or you upgrade to the pro version, than you can use the camera tool inside of TVPaint for the panning. But I would recommend to try the demo for the pro version first to see if the camera suit your needs.

Thanks to schwarzgrau . I think that answered my question .

You're welcome