How can I create .JPEG images in Adobe Illustrator, and will quality be maintained if I resize the image?
Make sure you set it to right size, or bigger than you think you need, as you can only make it smaller after you got a jpg file without loosing quality
In the file menu of illustrator there is an option to save as .jpeg or many other types of files. Make sure you save a copy for editing because when you save in that format you cant go back in and edit the layers.
Illustrator works with vector images, which are scalable (i.e. they retain their quality regardless of the size). You can export images to JPEG format (File, Save As... change file type to JPEG and set size and quality), but no, JPEGs are bitmap so by definition will NOT retain their quality if you change their size.
Continuing this (correct) answer the way to go would obviously be to resize your graphics to the correct size within Illustrator and then exporting to JPEG.
jpegs don't support transparency
How to Add an Image in Illustrator
I think you have to play with Resample Image in case of resizing image
Resample Image Illustrator
What you should know before placing images in Photoshop and Illustrator - See more http://adobephotoshopsecrets.blogspot.fr/2011/06/what-you-should-know-before-placing.html