Some sites in the Stack Exchange network allow images to be posted inline even by newly registered users. (For others, this has been disabled, as spammers and abusers also like to post images, and disallowing that has apparently helped reduce noise.)
When new users are allowed to post images, could the UI prevent them from also posting clickable links? And vice versa, I suppose -- if there are outbound hyperlinks, disallow images.
This should allow even newcomers to communicate graphically (this is perceived as important e.g. on the Graphics Design site; see discussion in this related meta post) but would deter e.g. SEO spammers, for whom posting an image is pointless then because they cannot drive traffic to the site they are promoting.
This will obviously not prevent spam altogether, but it will dissuade some spammers from posting, and constrain the options for those who continue to spam.
I'll note here that online marketers generally prefer to post ads with both images and links. Absence of one or the other reduces the response rate significantly in experiments - informally, text-only spam is less persuasive, and spam without links raises the threshold for those who would perhaps otherwise be persuaded to the point where they abandon clicking through. If you have to decipher a non-clickable link and type it in in order to visit it in your browser, or pick up the phone, or send a fax, many people will just not do it, and inhibit the impulse to go spend money on the advertised service or product there and then.
I tried to resist linking to XKCD#810 but of course, I failed.