are they normal file types? png, jpg, gif etc.. ? i've only used png and jpeg but assume others work.
if using some software to edit it, make sure it's saving as a generic file type and not something proprietary.
if you tink it could be a network issue.. resolve growdiareies ip address and run a /tracert on from command line. if you find bad hops, it might be a problem? i doubt this though.
make sure file sizes aren't inordinatley large relative to your upload speeds too.