Thanks for your replies.
I've made tests with the two maps provided by mib2berlin and bepeg4d. I can hardly find lots of differences, even compared to my original render, but I'm now seing watches everywhere...

I did mine using Gimp but I don't know if there's any option I should check, or may be use another software?
@ mlody47 : yes, I'll be interested in 'real' HDRi ready for use in Octane. I'm getting confused too with the different formats (polar, rectangular, panoramic, etc...) and I don't really know what is the right one and can't even make the difference between good and bad HDRi.