When I either attach an image (via the Attach toolbar button) or copy/paste an image, I get the same type as the original when I do so with tiff, jpeg, png, or gif. Why would someone go to the trouble of writing the code to convert a pasted image into some other, possibly larger, file format instead of using the original, when the original is in a format the application already supports?
I made an outline with a handful of images attached via either the Attach button or right-clicking on web page images in Safari, selecting Copy Image, and pasting into OO. I saved the file as images.oo3, and exported a copy as images.html (Dynamic HTML). You can see the contents of both here:
See any TIFF files?
Perhaps if you describe what you are doing in more detail we can figure out where your unwanted TIFF files originate, but I'm skeptical that OmniOutliner is creating them.