You didn't mention what you used to download the image.
Some browsers will automatically extract archives (our images are gzipped).
root@buildserver2:/mounts/files-rsync/osmc/download/installers/diskimages# md5sum OSMC_TGT_rbp1_20170129.img.gz
Looks like our MD5 is valid on our server.
Check as suggested above. If you still have a mismatch, re-download and if you still have a problem please tell us which mirror you downloaded from.
I suspect you ran a checksum on the extracted image rather than the images hosted on our CDN.