Does anyone know where I could find some open source art of some celtic looking heroes. Ideally more traditional art. Modern stuff is fine too, but I can't really afford to pay for it to include it in a jam game. :P

You can often find scanned public domain images from various out-of-copyright books on flickr. Here’s one set with Celtic mythological themes:

https://www.flickr.com/photos/internetarchivebookimages/tags/bookidmythslegendscelt00roll

The British Library put out over a million images from their collection, free to use, at https://www.flickr.com/photos/britishlibrary/. There's probably something in there is you're able to filter through it.

Thank you so much :)

I was going to mention the British Library collection too, which was my source of art for my entry... I should acknowledge that somewhere in the pdf, now that I think of it.