Further to this, you have constructed your feed in Podcast Maker from Lemonzdeam software. This application is seriously out of date and is no longer available. It's possible that if you started with an image of the correct size that Podcast Maker has resized it - 300 x 300 used to be the requirement at the time this program was current.
If this is what is happening the only way round it - other than using another method to construct your podcast - is after publishing the feed to go to your server and upload a full-sized copy of the image (with the same filename) to replace the one Podcast Maker has placed there. You may need to do this ever time you add an episode - I'm not sure whether Podcast Maker will over-write the image if you have not changed it.
You need to change the filename of the image and then go through this process - check on the server whether Podcast Maker has in fact resized the image. When you re-submit you may be told that the feed has already been submitted (this is a long-standing and annoying bug in the submissions process) - to get round this you will need to change the title of the podcast slightly. You can change it back once accepted.