All sites are going to limit you to either png, jpg or gif and it is in the design of the internet to produce low resolution images (72ppi by default).
I've never run into your problem when converting RAW formats to jpeg for later use on the internet. however, it is possible that your PS settings may be at fault. After you make your adjustments to your RAW image and open it in PS, look at it's size. It should be at least 300 ppi if it was shot in RAW.
The 'low res' warning may be resulting because the site is enlarging the image. When you save, what size is your image at the 300ppi resolution? When it uploads to the web, it is automatically converted to 72ppi so I don't see why the site would say anything about it being 'low resolution'.
Pro Photojournalist/Photoshop Guru