I downloaded some characters, but they are in Tiff format the readme says have a program that reads Tiff files, am i able to use tiff files in poser?

yep, poser seems to be able to load them just fine without converting them. converting them just cuts down on the size of the file cause TIF's are quite large and take up loads of memory.

If you use Photoshop, you can open the tiff files and resave them as jpg

The good thing about TIFFs is they can compress quite well but arent "lossy" like JPEGs, that is repeated opening, editing and resaving doesnt cause degradation. TIFFs also save an alpha channel when you render in Poser (so do PNGs), so the background can be made transparent in a paint package like Photoshop or Paintshop Pro - useful for compositing or multiple figure renders on low-memory PCs, like mine. When Poser uses them they probably use the same amount of memory as any other format but take more hard drive space to save. They are lousy at zipping, sometimes ending up bigger! TIFFs are our friends B)

Thanks for the info, Routledge. How do you make the background of a TIFF image transparent in Photoshop?

So just load the tiff like I would load a noraml jpg in the material editor no need to change it or anything?