I am not 100% sure I understand your response, it sounds like you are putting the images in an folder directly under your Sitefinity sites directory structure. I remember trying to do exactly this in SF 4.4 and not being able to get it to work. The solution I found for static content such as this dealt with the following folder structure found here
. For you to get this to work, I think you will need to do what I suggested previously, (albiet badly).
1) Create a folder under App_Data/Sitefinity called WebsiteTemplates
2) Create your images folder under website templates (or however you want your directory to look like)
3) Add your images to this folder and then reference them in your sitefinity site. You can do this using the "~" pathing or through relative pathing.
The reason this isn't working for you in SF 5.0 is that SF is managing all of this static content for you. If it is not found in the website templates folder it isn't able to manage the static content like images.