You can edit the campaign file directly.
Make sure FGU is closed.
Find the campaign folder. There is a folder named "campaigns" under the Fantasy Grounds folder. Under that folder is a folder named after your campaign.
Inside the campaign folder is a file named db.xml.
Make a backup of db.xml!
Open db.xml in a text file editor like notepad.
You need to find the following xml nodes. Child nodes are indented.
Look for <partysheet>. Under that is <image type="image">. Under that is <layers>. It will look something like this:
.....lots of other nodes...
<partysheet>
<public />
.....more nodes...
<image type="image">
<allowplayerdrawing>on</allowplayerdrawing>
.....more nodes...
<layers>
<layer>
.....more nodes...
</layers>
</image>
.....more nodes...
</partysheet>
The bold pieces above is what you want.
Replace the entire chunk of layers with <layers />. So <layers>......</layers> becomes just <layers />. That will be a fair number of lines.
Those are all the items drawn on the party sheet. If you look at the contents of the various <layer> child nodes, you'll see names of the images you dropped in there. That's what you want to delete.
Save the file and close it.
Open your campaign in FGU and check the party sheet.
If you mess up the file, close FGU, delete the edited db.xml, and copy your backup to a new db.xml and try again.
Keep the backup until you're sure the edited campaign file is good.
Ask for help if you run into trouble.