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Usually a "disk image", is a ".sparseimage" (or ".dmg", or ".sparsebundle", etc.) file. Such a file (or bundle) is not directly bootable. The problem is that the boot firmware of Macs only really supports reading files from a proper disk volume (disk -> volume -> file). But involving a disk image puts an extra layer in there, and the firmware does not handle that situation (disk -> volume -> disk image file -> disk -> volume -> file).
But a disk image can properly store all the data required to recreate a volume that is actually bootable (PPC: internal/FireWire, APM partitioned, HFS+ volume; Intel: internal/FireWire/USB, GUID partitioned, HFS+ volume). Among other details, it is this extra step of "restore before boot" that makes backups to disk image less desirable.
In any case, disk images can be copied by Finder or just about anything else and still be used to recreate a bootable volume (just use SuperDuper! to copy the disk image to a suitable volume).
If by "disk image" you mean a backup made to a volume on an actual disk, then, no using Finder to copy the files from this volume to another will not result in a bootable volume. Such a copy will likely end up missing much of the metadata required for a properly functioning system.
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