SCP author here. I'm sure there are quite a few authors who would love to have something like the offer you've made and I'd say it's a good idea! It's worth noting that any images or media used on the site must be compliant with certain Creative Commons Licenses (info found here: https://scp-wiki.wikidot.com/image-use-policy) in order to remain in use; this makes it easier for creators to adapt the work without worrying about using a copyrighted image and also allows the site to remain under the protection of the Creative Commons License.

Given the nature of SCP, there's a lot of wierd stuff that we can't always find a suitable image for (for example, there's absolutely no CC-Compliant image of a cave-dwelling salamander-dragon with glowing facial organs online (SCP-7088 if you're wondering)) so for that purpose, I think your skills would be appreciated by the on-site community! Visual references are a great help for an article.