|Is there a fix for this or do I have to image from a dual core?|
Yes you will - and if you try to use that dual core image on a single core computer it will probably crash.
This is because the HAL (Hardware Abstraction Layer) file is modified during the installation to reflect the hardware that XP is being installed on. The HAL file is only expecting a single core and that is all you get.
Images can only work when installed on computers with identical hardware that the original image was made from.
You may get round the problem by doing a repair install on the duel core computer where the installation should pick up the second core and modify the HAL file accordingly.