Where you're hitting the problem is "...exported it to Photoshop." Don't do that.
One of the great things about working with the Creative Suite applications is the way that they can work together, and this is a typical case where working the "old" way (export to a format that the other program understands, then import) is actually counter-productive.
Save your Illustrator file in its native format, then either drag it into your Photoshop banner document or use File > Place.
In either case, the Illustrator file becomes a Vector Smart Object, which you can scale as much as you like. Pixels are rendered on the fly by Photoshop each time you change the size.
As a side note: most of us grew up in the world of individual applications with proprietary file formats and fragmented workflows. Getting past that mindset and starting to think of the different applications as active collaborators, rather than talented but socially-inept loners, is essential if you want to utilize the full power of any them.