tamam buldum
custom fields'e "Thumbnail" girmen gerekiyormuş

baş harfi büyük olarak. value kısmına girdiğin resim otomatik olarak yeniden boyutlandırılır ayrıca resimlerin tutulacağı phpThumb klasörü chmod 777 olmalı.
Kaynak : egallery readme dosyası
eGallery utilizes a script called phpThumb to automatically resize images. Whenever you make a new post you will need to add a custom field. Scroll down below the text editor and click on the "custom fields" link. In the "Key" section, input "Thumbnail" (this is case sensitive). In the "Value" area, input the url to your thumbnail image. Your image will automatically be resized and cropped.
Important Note: You must CHMOD the "cache" folder located in the phpThumb directory to 777. If you don't do this then the script will not create cache images which will result in a huge load time increase as well as a bunch of uneeded temp files. Just do it, or your server might get a bit clogged up.