Are you trying to insert an image into the HTML of your listing? That's about the only way you can have really large, clear images in your eBay auctions. eBay's image hosting always adds compression and tends to truncate the overall size of the image.
The code in the auction should look like this:
<img src="
http://i4.photobucket.com/albums/y135/w ... t.jpg"></a>
That's an example using an image hosted in my photobucket account. Imageshack and Photobucket are free image hosting sites.
Let me know if that helps. Or if I even answered your question.