For example what if I take a picture of a Tonka truck, can I use that as an art asset? What if the toy is more generic?
Usual caveat: I am not a lawyer, this is not legal advice, the relevant laws vary from country to country, city to city, etc.
The basic concepts are these: you own one instance of the truck. You can sell your instance of the truck to someone else (according to the first sale doctrine, which states that the Copyright owner's distribution rights only apply to the first sale of each particular instance of a protected work). You may not create new instances of the truck -- those reproduction rights are still possessed by the copyright owner.
What you're asking here is whether the copyright owner maintains his adaptation rights -- the ability to take one work (a model of a truck) and use it to make a different, related work (a photo of a model of a truck). The law here is a little less settled. We could point at different cases which have ruled in different ways, about whether or not the copyright owner surrenders his adaptation rights at the same time he surrenders his distribution rights, via the first sale doctrine.
WIPO's position on the topic is that one needs explicit approval from the copyright holder in order to adapt a copyrighted work into another form -- that the first sale doctrine only applies to distribution rights, not adaptation rights, but even they acknowledge that the issue is unclear at the current time.
Best advice is to ask a qualified lawyer who practices in your area, who knows the relevant up-to-the-minute case law, and how it's interpreted in your area. This really isn't the sort of thing that you want to get wrong. :)