While it probably technically isn’t to hard using dropbox/googledirve/onedrive/icloud (or your own owncloud) to distribute such build, I’m not sure if you’re legally allowed to distribute the *.img if you have included the binary blobs from wget http://code.fairphone.com/downloads/FP2/blobs/fp2-sibon-2.0.0-blobs.tgz as their special license agreements – to which you in that case have agreed! – states:
\2. Fairphone grants you a free of charge, non-exclusive, non-sublicensable,
non-transferable, limited copyright license to download, install and use the
Software for non commercial purposes only on a Fairphone 2 device in
machine-readable (i.e., object code) environment. You are not allowed to use
the Software in any other way. You are also not allowed to remove portions of
the Software, alter or otherwise modify it, or translate, reproduce, copy,
reverse engineer, reverse compile, disassemble or transfer the Software.
(emphasises by me)
Especially as any person downloading it would not have to agree to the terms themselves.
Note: I’m not a lawyer, so this is only a naive, personal opinion of me.