1. they don't show on the first page (frontpage) on my Windows machine .
2. on my Edubuntu Linux machine they show 3. on my friend's Windows machine they show correctly
Sorry, apparently I didn't understand your question correctly.
Did you check the response headers of the HTTP requests for the images? What if you access the images directly with their URL? Did you try to download them and open them with another application? Maybe some app is blocking your images. In either case, this is probably not an eZ publish-related issue.