Hello what format would be best to improt a background from another software ?
It will really depend on what your image consists of (gradients vs. solid fills), and what you are looking for. Some compression (ex. jpeg vs. png) will work differently on different types of images. The most important thing to remember when importing bitmap images into TBS is to make sure to have the correct resolution before you import it. Zooming into a bitmap image in any software, will become pixelated if the resolution is not there.
Hope this helps,