I have a Clonezilla image of a 250 GB disk. Are there any caveats or downsides if I would restore this to a 500 GB disk? (I will of course not be able to use more than the 250 GB on the new disk, but this is not a problem.)
No, there are no caveats. It will just work. For most filesystems (including ext3/4, xfs, ntfs), clonezilla is even able to automatically expand the filesystem so that you get to use the entire 500GB.
In fact, this is one of the benefits of Clonezilla. In a previous job, I set up a workstation cloning system using clonezilla where we'd make the clonezilla images as small as possible (e.g. 10 or 20GB) in order to minimise network traffic and time required. The images were installed on new machines with 500GB or 1TB drives or larger.