When inside Poser and you click "export" for the image, you will get a "save as" window.  Where it says "Save as Type", use the drop down arrow to select the file type (extension) of your choice.

The fact that it's saving with .png at the end, tells me that you have the "Save as Type" set to .png and are typing in the name of the file along with whatever extension you want IE:  myimage.tiff

No need to type in an extension. Just type in the name and Poser will assign the appropriate extension automatically, according to what you have it set for.

"It is good to see ourselves as others see us. Try as we may, we are never
able to know ourselves fully as we are, especially the evil side of us.
This we can do only if we are not angry with our critics but will take in good
heart whatever they might have to say." - Ghandi