Running this operator and and closing Blender gives this:
Error: Not freed memory blocks: 2
ImBuf_struct len: 2480 0x69ba4f8
imb_addrectImBuf len: 1048576 0x6ccc2d8

Fixed with added call to IMB_freeImBuf in BKE_image_add_from_imbuf.
Could be fixed in the operator instead, but I think the BKE function
is the correct place since the comment says it should take ownership
of the ImBuf.