Welcome to Auctiva!
If you close your InkFrog account, I believe the InkFrog hosted images that are currently being used in active listings would no longer be present in those listings. However, I am not an InkFrog expert by any means, so I suggest you contact their support department regarding this issue as well...just to be sure
While there isn't an "easy" way to convert these listings to use Auctiva hosted images, this can be done by revising each of your current listings that uses InkFrog hosted images via the following process:
1) Recreate the listing in Auctiva and click "Save".
2) Once you have finished, click the "Save" button.
3) Return to your Saved Listings page, check the box next to the item, and click the "Get HTML" button.
4) Select All and copy the html in the popup window.
5) Go to the item that is active on eBay and click "Revise Item" then "Edit Description"
6) Click the "Enter Your Own HTML" tab above the description editor.
7) Delete the html that is already there and then paste the copied html in its place and save you changes.
Another option would be to begin creating your new listings with Auctiva listing tools and image hosting, and to wait until your listings that currently use InkFrog hosted images expire to close your InkFrog account. This is probably the easiest option but it may require paying for InkFrog's service a bit longer.
-Mike