GIF is restricted to 256 colours. If your image has a lot more than that, it will look anything from not very good to appalling.

With JPEG, the critical thing is the degree of compression. JPEG loses data to achieve the levels of compression it does, so if you have a high compression set then your images may look poor. Are you using Poser - this by default sets a very high compression (i.e. low quality)? A better bet is to save as a TIF, PSD, or PNG, then convert to JPEG in a bitmap editor like Photoshop or Paint Shop Pro, which will give much more control over the compression process and give much better results.

Steve