Problem with changing the thumbnail

I tried to insert a Journal thumbnail and I click save. When I go to website preview, It still shows the old PKP Journal thumbnail. I refresh the page, cleared data cache, clear template cache but still, the problem exists. I even log out and logged in several times but still showing the old Journal thumbnail.
I am a new user and I am using OJS version

Same here - after migration to OJS 3.2.1-1 (from 3.1.2-4), some thumbnails are not shown. Also, when one tries to upload one, only PNG is accepted, and in some cases, even not this.

@asmecher: This is entered in the database upon uploading a journal thumbnail:

select * from journal_settings where journal_id=7 and setting_name="journalThumbnail"\G
*************************** 1. row ***************************
   journal_id: 7
       locale: de_DE
 setting_name: journalThumbnail
setting_value: a:0:{}
 setting_type: NULL

OK. Deleting the cache did help - but still only PNG seems to be accepted.