Any data can be represented using bits (bit is smallest unit of information, 0 or 1).
'bit pattern' is a sequence of bits, in a memory, a communications channel or other device. It is of predetermined length.
'bit string' is similar to 'bit pattern', except it is of arbitrary length.
in context of this blog (at least), 'image' is similar to 'bit string' or 'bit pattern', but it is more or less accurate representation of something. for example: complete disk image, incomplete disk image (unimportant bits omitted), compressed picture image (with or without loss of quality).