There are 2 main reasons for this problem.
If the version is quite recent (> 6.x) you'll notice that the error happens only with png images and can be solved simply using absolute paths in the first parameter of $pdf→Image().
The second problem could be tied to the permissions on cache and images dirs (if they exists). In that case set them to 777 and try again. If ok, reduce them to 755 and try again.