Swap thumbnail for full on hover

I’m trying to write a script that changes the banner src when you hover over a thumbnail. In my theme I’ve got two custom image sizes a 90×90 thumbnail & a banner image size which could be 9999×230. I thought I could manipulate via jquery the image src of the banner by removing the 90×90 in the src attribute. This worked, however it is generating the full size image instead of the custom image size with the possible 9999 width x 230 height. Any suggestions on how I could go about pulling in that correctly sized banner image?