Social Paid ads and having incentives for users to upload. I've struck a couple of deals with some dive suppliers who in exchange for free advertising offload their runout products for me to give away - but it always seems to be the same users uploading over and over, so it'll be more paid ads on FB
Our method is pretty simple. We already own a Celeb fitness forum which is mostly discussion based and media driven (pictures & videos etc). Thus, we thought it was a "no-brainer" to lobby our own image-hosting service. Luckily, I was aware of Chevereto cms and it's been around for years. It's a cool product with many features and a hardcore on-going customer support.
What we're doing with our forum at the moment is gradually building an internal program whereby any 3rd party image-host links are converted to our own chevereto cms links.
That's our route to marketing our own image-hosting service. We're basically using our own community to lobby our own image hosting service. It's still in it's infancy and I am kicking myself for not doing this earlier.
Our image hosting service is aimed for our forums member use only so we don't need to market that as such.
Like ijayen we are also using our host to re-upload images from 3rd party hosts plus use it for new uploads daily, as we are a community that holds weekly competitions having everything on one host is the main reason we decided to host our own images as having one of the 3rd party hosts down can ruin the voting process altogether.
The majority of our content also gets posted on many other forums that are similar to ours so the image host in it self is basically a way to drive new members to our forum, when they open up a thumbnailed image and see it is hosted by our forum they then visit and join up.
What we are doing is probably the complete opposite of what you are trying to achieve, by not using the image host as a means of funding it but to use it to drive members to our forum where they willingly help donate to the hosting costs after they get hooked on the forum.