MicrostockGroup

You dont need to do that. They upsize and convert to tiff you file for buyers.

I wrote a letter to DS support  and they answered "The only additional formats are the ones below. Please see the link:
http://www.dreamstime.com/extensionstable.htm" but the big amount of 3D renders in DS have TIFF format for the Maximum size.

It may have something to do with the resolutions of the images you are submitting - the TIFFS are generally the result of DT's upsizing and you may be at the upper end already.  I have a vague recollection of a thread on this......

DT only says that they use "special software" to make the conversions. I believe that they are using some type of fractal program (similar to Genuine Fractals) that "transforms the image into "resolution independent-assets" eliminating the relationship between pixels and resolution. The image becomes mathematically encoded as an algorithm and the pixels of the original raster image are replaced with a new file structure that stores the entire image and none of the pixels. When you open the image again, you can re-scale it to the desired size and the algorithm will generate new pixels while maintaining sharpness regardless of image size."

It must work if buyers aren't complaining.