Your menu is not pointing correctly to that animal category. It is looking in /category/animals/ whereas your hippo and coon are found in /image-type/animals-2/. I have no clear idea what might have caused this, though...

Thank you, Stephan, for spotting and pointing out the difference.

I thought, that I processed the animal images in the same way as the others. One possible difference is the SLUG. Slug for animals image keyword is "animals", whereas slug for animal image category is "animals-2". Does anybody know how are the slugs used and where are these definitions stored?

It has been discussed before somewhere that the keyword/category slugs need to be unique. In case you have keywords that match your category names trouble starts: WP automatically adds a "-2" to the later slug with weird outcome. e.g. a keyword with a slug such as animal-2 wouldn't show in search for "animal". Your case is the other way around. I can't remember having read someone else had issues like you but it could very well be the root of your problem.

To avoid such conflicts I have renamed all my category slugs to "image-category-whatever". I think this little extra effort has saved me a lot of headache.

Maybe try this:
Edit slugs of ALL categories to something like "animals-images", "nature-images" and so on, to avoid conflicts with keyword slugs.
Then go to Stock Images->image keywords in admin area, find and edit all keywords slugs, that end with -2 or other number. Change slug animal-2 to animal, nature-2 to nature and so on.

Thanks, Stephan

1. It was indeed in the the Menu definition. All menu items had "Image Categories" in the Menu Item field, whereas the Animal line had "Categories" as the Menu item. I can't remember what I did differently and where the "Category" came from.

I deleted the old Animal image category at menu item, created a new Animal Image Category, and then added it to the Menu.  It works properly now, and it picked up automatically all existing images which were defined in Animal image category.

2. As to the possible mixup between the image keywords and image categories with the same name, now, I don't think that was part of the problem. As long as there is at least one character different in the corresponding two slugs, the system will properly differentiate, so you don't have to rename the slugs.


Amanda, if you want to add a Troubleshooting section to your Symbioguides, you could mention this situation and fix there.
 

1. Just to clarify - my problem was related to the image categories drop down or image category indexing - not to the image keywords.

2. I wasn't aware of any problems with Search. I couldn't find any old posts describing this problem. Can you point me to those posts?

I just tested the search with my limited image collection, and couldn't find any problems. As far as I can see, the Search will look at the keywords only (exact wording).
For example, if an animal picture is in the Nature category, but was not keyworded with "nature", the search won't find. However, you would find it in  the Nature image category. After adding the keyword "nature" to the image, the Search will find it.

However, there is a problem with singular and plural forms.
If I specify "animal" as keyword, and then search for it as "animal", the Search will find it. If I search for "animals", I am out of luck.
How should we deal with this problem? Include both plural and singular forms in the keywords, or hope that the Search will evolve to be able to recognize both forms?   

3. What happens if you rename the slug from nature-2 to something else halfway through your uploading (or a year from now)? Will you have to delete and reupload the related images uploaded prior to the slug change?

4. Lastly, is this indexing structure included in Leos' Symbiostock theme or is it an integral part of the wordpress (taxonomy and keyword/tag indexing)?