Intermediate sized random photos

I am a novice to PHP. I have recently installed Gallery v.1.4.4-pl4 embedded in PHPnuke 7.4 with random block for random block for 1.4.3 from nukedgallery site. It works well but I have a problem when I tried to modified its behaviour.

I tried change the random picture size to 480 following the config instruction in readme file. But the site only display jagged images, which seem to be simply enlarged from the thumbnails.

I want to have a random block to display one intermediate size photo for each visit on the phpnuke home page so that it can be more eye catching when people see the first page. Could someone tell me what should I change in the block file? Thanks!!