Adding Image covers to galleries may not be as automatic as you think. There's a small glitch on the site. This is actually an issue about dealing with a big of a bug on the site. Normally, I would write these up in a post for the Bug Report section of the forums, but those don't often get taken care of too quickly. Not that I doubt the staff can't or wont. It's more a matter of having bigger fish to fry, as it were.
I've found some work-aronuds that should help you out. I'm going to use the Dakara Boku-ha, H ga Dekinai manga page I added recently as an example.
Firstly is a pretty minor issue. When creating a new manga series, you should see that the cover image wont appear on the main series page.
This is an easy fix. Just go to the first volume and fill out the release date. Go back to the main page and you'll see the cover image.
Some of you may or may not have noticed this next one. Here's bigger problem. I'm seeing users adding a lot of new volumes to the series page, and that's great. The problem is that there is a bug on the site that doesn't sent the images of ever cover you add to the Image Gallery of that series. If you look on the image below, you can see that there are ZERO Images in the gallery when I mouse over it.
Since there are no pictures in that gallery, if you were to do an image search for this series to say use the cover for the wiki page information. It would show no images in that gallery. Up can't easily search for those images anywhere. The only place they are in now is in your own personal gallery when the user who uploaded them.
In the left had tool bar, there is no obvious way to get into that Image Gallery. Here's what you need to do. Just click on the cover image and it sends you to the images. You can see it in the picture below.
Just add the picture from your personal images into the gallery. It's now searchable in the gallery for that series. If you go to the gallery now, you can find the images for both covers in the series. That's why you can always search for series I work on and find all the covers I've uploaded.
If you've been adding volumes all this time, Those pictures aren't in the series gallery.
If you've been adding a lot of covers and you don't like the idea of trying to dig through all your pictures. Just go into your profile's Image Gallery and create an Album called Covers. That way you can add all the covers into in an album that's easy to get to when adding pictures to galleries. This will make searching faster.
Just don't delete the images from that gallery when you're done. That can sometimes remove the image from your personal gallery completely. Just delete the album.
Here's another bug that's easily fixed for users. The first image of the volume added are never sent to the Franchise page, providing there is one. Though oddly enough, any picture added to that volume page will be sent to the franchise.
First, you need to make sure the franchise is linked on the main series page.
Even though the cover image already appears on the page. Hit the "Edit This Page" on the individual volume. Above the cover you you'll see the next "Upload images for this book". This is how you add additional cover images. Just select this again, and add the original cover image once more. That will send it to the franchise page.
This also automatically created a "(Manga)" album for that series in the Image gallery of the Franchise Page.
Maybe some day the staff will be able to fix some of these issues. In the mean time, these should help you work out these issues.