The image in iTunes is directly linked to an an image you have either uploaded to yoru website, or your media host. If it's not showing there are typically a few reasons these inlude:
1. You just submitted your podcast, and the podcast ist listed but the image is not. This is a "no big deal" situation. Give it a week. For whatever reason when your podcast is first added to the directory, your image doesn't always show up. In some cases the image that appears from a search is missing, the image on your acutal listing displays.
2. You accidentally deleted the file that iTunes is looking at. If you are using the PowerPress plugin click on Powerpress, then click on the iTunes link. Scroll down and look for the Itunes Image section. Click on the Preview link. If it doesn't appear then you need to upload the image again.
3. If you are using Feedburner go to the optimized tab, click on Smart Cast, you will see where it has a section for Podcast Image Location. Here again you have a "Preview" link. If you click it and it doesn't apear, you need to upload a new image and put that location in this feedburner section and save your changes. ( I do not recommend using Feedburner)
4. If you click the link and the image appears, then your end of the technology is OK.
5. Another alternative troubleshooting technique is to go to your RSS feed and choose to view the code then look for the iTunes image link. Here again, copy and paste the address of the image. If it appears, the trouble is on Apple's end.
If the trouble is on Apple's end you should contact their support (or report a concern in iTunes). As podcasting doesn't bring the company money, it is not always their top concern.
You can try emailing firstname.lastname@example.org