At the moment artists cannot post unless the images are embedded within a document or on a particular url. HOWEVER this is a very good time for artists to join because AO3 are always looking for suggestions and preferences for the design of their upcoming fanart galleries. If you wish you can become an active member of the design crew for the galleries or a beta. Most of the proposals have already been submitted but they seem to be open on an ongoing basis to suggestions. They are working in cycles of design and will be shortly looking for betas for the galleries. See here for information.
They are running a similar project for vidders here if you want to read details