I tried to put an object into my last scene and it showed up without textures. The textures are there like in the same folder...etc. What is the trick to get TG see and actually use those textures? (This object was downloaded from the internet with the textures)
Import it in Poseray and see if all textures are read properly, save as obj and try again in TG.
Okay I will get that software. Otherwise if Poseray shows all good then something needs to be done in TG right?
You might need to reload some textures in TG, indeed. In the default shader inside the object parts. Especially the opacity input won't always be there (in trees, e.g.).
Frankly, I'd just go with re-texturing, leaving additional software behind. Recently I started playing with DAZ 3D and all objects that I export from there and load into TG are entirely glossy-black. What I do is just specify path for each 'default shader' that I'd like to use. Once you do it you can save your TG project and the next time you open it all textures will remain in place.
If you want, you can also try out an "assembly line" idea of mine. I created a TG "forge" file where I prepare, resurface and .tgo-save all models for later use. When I specify texture paths there, I never have to resurface models again even if I put them in various files! Don't know why and how, but it simply works.