I'm getting the "This page contains both secure and nonsecure items" message on my site. I've identified what's causing it, but I'm not sure how to get around the problem.

I have a sidebox on the right called "Why Choose Us" which contains some images. These images are absolute linked to a subfolder in the main images directory, using a http. On the left side, I have the Newsletter Subscribe mod (which uses SSL on its pages). If you click on the subscribe button (can leave the email address blank), it throws the nonsecure warning.

I'm not sure what to do. The images in question need to somehow be configured to work on both SSL pages and normal pages.
I tried changing the links to the images to https but I then got some warnings when looking at non SSL pages. I'm not sure whether a relative link would solve this?

It's not a problem at checkout because I disable the right side column during this process. It's just the Newsletter mod which is causing the problem.

Can anyone help?