Just a million questions to get some more details and maybe help solve the problem;
- What Tiki version are you using?
- Are the images stored in the file system or the database?
- Have you tried the rebuild thumbnails option?
- Has the server had a change of PHP version and is not maybe missing GD?
- Whats the memory_limit on the server?
(there is an article http://tikiwiki.org/tiki-view_articles.php to give details on checking and setting this)