Aperture adds another layer of complexity when dragging an image from it or using Flux's built in Media Browser.
Aperture generates previews of your photos in the Aperture Library that are smaller in size than the originals, it is these previews that are used when you drag an image from Aperture or use an image from the Media Browser.
If you take some or all of your photos using the RAW file format the preview you drag in will be a jpeg. This saves you the trouble of converting the file type.
Setting your Preview quality too low will cause poor image quality on your displayed web page. You can always resample the image in Flux.
If you need a large image within your site or a high definition alternative then you will have to use the Aperture Export controls or set your Previews preferences differently.
Check your Aperture Previews preferences.
For more detailed information on using Aperture read "Aperture 3 Portable Genius by Ellen Anon".