podcast cover art not displayed anywhere

Yes, less than 500kB! Which your image is. However, you do have an alpha channel. I don't know whether this is an issue, but to be on the safe side I should flatten it. I also don't know whether the fact that the filename is in Amharic is an issue: again, my instinct would be to change it for something in Roman characters.

The image which is displayed in the iTunes Store/ApplePodcasts Browse is carried in the 'itunes:image' tag in the top section of the feed (before the first 'item' tag). The images embedded in the ID3 tag in the media files appears only when subscribers play the file (and them very small).

It's normal for it to take a few days before the image appears when a podcast is new: but of course if there is anything in the parameters which is incorrect it won't appear. 3000 x 3000 px is fine, but the file size must be less than 500B, and the format JPG or PNG, RGB, with no alpha channel. I can check the image if you post the feed URL (which it's advisable to do whenever you have a question).

You have a commented out line of html at the end of your feed:

<!-- permalink_structure ends with slash (/) but REQUEST_URI does not end with slash (/) -->

which may or may not matter but it is technically inadvisable.

One of the validators objected to the appearance of the html code &#8230; in the 'itunes':summary' tag foir your episode. This shouldn't cause a rejection, but it may well not be rendered and may even be displayed: as it's the code for an elipsis I should replace with three stops: ..

Minor details: your first category (the one which will show) is 'Technology'. You have 'Tech news' as a sub-category: there is no such sub-category and indeed Technology does not have one.

Your second category coding is incorrect. It reads

<itunes:category text="Education">
<itunes:category text="How to"></itunes:category>
</itunes:category>

it should read

<itunes:category text="Education">
<itunes:category text="How to" />
</itunes:category>