Something I recently came to realize when importing tokens with saved assets to a campaign without those assets. The Player will see the image just fine, but everyone else will see the big red X. This just means you have the asset on your computer, and the others don't. What you can do is drag the image from your library on to the map. The asset will be copied to the other users and your red X is replaced with the proper image. You can now delete the dropped image or have the DM make an image token out of it and move it to an appropriate area. Doing that will assure the image is available to any new users. I'm not sure if the DM asset cache would be transferred, but I'm guessing it's not.
What you could also do is create your own online repository of images, then give that to your DM to add to the campaign repository. You can create one of those by dropping a bunch of images on a map, then using the export feature to create a Campaign Repository File. Then upload that file to the web. I use sugarsync
to get free online storage (5 and 2 GB respectively). The best part about giving out a repository file link with those sources is that you can later update it and the campaign will be updated automatically without GM intervention (assuming you replace the file with the same named).
Admittedly, I think a repository is a bit of an overkill, but that will assure your images will be seen by all with the least amount of GM manipulation.