There are a few things that may be going on here. Your original web hosting package may have included a permission to use the photo but that permission did not extend to other web hosting services. Or, perhaps the first web hosting arrangement never really got the proper permission, so wherever you use it, you'll have a problem. The photo may have some digital watermarking embedded so that the stock photo company can trace all such uses and catch those who use it without permission. If you're using it under the terms of the original webhosting agreement, you need to look to your agreement and your webhosts for resolution. If not, the stock photo company likely has a legitimate claim for infringement.
How far will they pursue it? It's possible they could file a lawsuit, but that doesn't seem practical or likely -- they wouldn't recover the costs of the litigation. Odds are good that if you write and explain your mistake -- you mistakenly thought you had a license to use it -- the whole thing will go away. They may send some more threatening emails, but the chances of anything going beyond that are unlikely. If I'm wrong (hard to believe, but it happens), let me know and maybe we can trigger a Streisand effect.
