I have a website with a lot of images on the front page, and in periods of heavy traffic I get 503s. I'm want to host these images on another server (or at least some of them), and I'm looking for providers that can do this without charging to much (or even better, for free

Any ideas?

I'd say you would need to upgrade your hosting arrangements, if you're on a shared server the load is probably too much during your busy periods. Hosting images on another site can just slow your site down so you'd be replacing one problem with another.

I would try imageshack.us I've hosted some images there 2 years ago and they're still up!!