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 [nofollow]" but the big amount of 3D renders in DS have TIFF format for the Maximum size.

to:Suljo   

So, why in my portfolio in DS the  pictures with the Maximum size,  for example: 8000x5000px (7279 KB) are not with TIFF license ?

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.

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.

It exists different softwares to enlarge artificially bitmap images. And they are in nothing "special"�
But no one will give you a real good quality (seen on your monitor at 100%).

If you image is 100 x 100 pixels you have 10000 pixels.
If you resize your 100 x 100 pixels image to 200 x 200 pixels you will still have the 10000 original pixels + 30000 pixels generated by interpolation (30000 fake pixels�)
For example is you have a white pixel with an adjacent black pixel, when you will enlarge artificially the image, the software will interpolate the values and create a series of gray pixels in between.

Generally these software use to had some artificial noise to soften a little the image.
The final result is never very good. 
But I have to say that in most of the cases if you will need the image for offset printing you will get an enough good result, as good as the one that you can obtain with a noisy image.

If you have to enlarge your image up to 20% use the "Image size" in Photoshop.
Or if you use the image for printing in InDesign of other DTP softwares just resize there your image (the correct interpolation should be done during the generation of the pdf for printing).

Convert a jpeg in tiff is absurd.
Enlarge an image more than its original size does not make much sense.

^^^
Oh yeah