How would I transition the opacity of an image from 100% to 0%?

You can do this with the Transparency panel in Illustrator.

Draw a shape over your photo that you want to use for the mask.

Select the image, and the shape you drew over it. Then in the transparency panel click to create the mask.

Now with the masked image selected (in my example there was a rectangle the same size as the image). Click on the mask thumbnail in the transparency panel. Now you can fill it with a gradient swatch, and after you choose that gradient fill you can use the gradient tool to adjust it's position, length, etc. A black to white gradient mask will give you the blend effect you are looking for.

Illustrator is an Object-based applocation, so to make the transition (hiding) part of an image into another you need to use the Opacity mask which includes drawing an abject with Black/white gradient and use the opcaity mask.

otherwise, Photoshop is the Ideal environment to deal with Pixel-based files, just add a Layer mask and use the gradient tool with black/white