It's unclear to me what you're asking for, but there are only two track elements that can be handled in a paint program: the terrain textures (common) and the terrain itself (rare).
Creating your own terrain textures and slicing them up into tiles can involve a lot of planning, multiple steps, and there are several different ways to go about it all. It's basically a one way process - you create a large composite image in a paint program, you slice it into 64x64 tiles, then you assemble the tiles on the terrain in Traxx.
As for the terrain itself, it's best to use Traxx to carve it but you can
create or edit it in a paint program since it's actually a 256x256 grayscale (8-bit / 256 color) image, where white is the highest altitude (palette value 255), black the lowest altitude (palette value 0), with 254 shades of gray in between. If you're in terrain edit mode in Traxx you can copy and paste the terrain as a bitmap (to and from your paint program), the option is available in the Traxx edit menu or with a right-click.
A great old resource: http://mtm2.com/~mtmg/traxx/