Assume that the image is protected by copyright, and there is no license to copy it. I assume you are a programmer who would be assembling a web page for the client, and you have a copy of the image, let's say on a thumb drive. What you would do is make a copy from the thumb drive, to a server. Without authorization of the copyright owner this act would infringe the copyright. The client might also be liable for contributory infringement.
It is legally pointless to say that you assume that all material is "okay": you cannot disclaim liability for copyright infringement that way. It may be possible to disclaim liability for damage to the client, but it is impossible to disclaim liability that you have to a third part in this manner. Instead, contracts typically have an indemnity clause, which states that the client must protect you by covering your legal costs and whatever damages you are assessed, if their wrongful act lands you in trouble.
Of course verifying that the client has the right to use the image is also the simplest way to avoid lawsuits.