I only see a few IR photographs from you posted on this board. So it's a little tough for me to judge your photo quality.
But I will mention that it's tough to compete with istock in a way.
They never have to worry about paying for royalties, which makes it very appealing for most companies.
I have sold some web sites, and a book author the rights to use some of my work in a project. However I get more advertising than anything out of the deal.
I can usually only charge them what they'd pay the royalty free sites if I want the coin