These are fully spherical environments created using Dreamscape by Sitni Sati. The resolution should be sufficient for most purposes, such as generating a quick environment suitable for TV resolution with a "human eye" FOV.

To safely download the high-res versions to your harddisk, right-click on the images and choose "Save Target as..."

To seamlessly integrate these panoramas with you scene, you might want to use a layered fog (MAX atmospheric effect) in addition, with instances of your environment texture as "Environment Color Map" for the fog. Thus the pre-rendered horizon and the ground level of your scene should blend seamlessly. Also you will get more natural results if you map these environments on a relatively large sphere surrounding your scene, rather than using them as MAX environment map only.

Add realism yourself by placing a light source and/or a superbright object at the position of the sun (even for the case that you want to do pure image-based lighting only, which these maps aren't really suitable for). In dark reflections these skies won't look very realistic, since the seemingly overbright parts of course don't exceed the RGB spectrum and hence will look "clamped" as well. This will be most obvious for darker reflections of the sun itself, hence you should create a light source for an additional specular highlight or a very bright object for a more accurate reflection. Optionally you might also want to add a lensflare, but please don't exaggerate it ;-)
Also these maps aren't post-processed in Photoshop yet. I leave this up to you, because it would only make sense for the finished rendering, anyway.