Assuming you use Photoshop and have a document size limitation try this

open you image in Photoshop and then go to the image tab and choose resize image size and then untick the resample image button and in the resolution put this size in 30000(just the number) pixels per inch then tick resample image and resize to the number you need and choose this option from below Bicubic Sharper (this is best for reduction) and you image should be at is best save it either as a .jpg or even better .png file (I don't know if renderosity accepts .tiff files but if they do use a .tiff as this will make it even better than a .png

Hope that has helped you

Regards

Karl