to answer your 'why' question ... the image is displayed as a thumbnail, with the intention being that clicking on the thumbnail would display the image in full size.
Mitchell was correct, if you don't want that behavior, edit the template.html file for the repository skin you're using and remove the hyperlink that displays the full size image.
Here is the recommended way to edit templates so that your modifications are not overwritten on upgrades.
1. create a folder named "RepositoryTemplates" in your Portals/n folder. for example, if you're working in portal 0, create a Portals/0/RepositoryTemplates folder.
2. copy the template you want to modify from DesktopModules/Repository/Templates to the new folder, for example, if you are modifying the default skin, copy DesktopModules/Repository/Templates/default to /Portals/0/RepositoryTemplates/default
3. modify the files in the new location.
The Repository module will always look in the Portals folder structure first for template files, then if it can't find them there will look in the DesktopModules folder structure.
That way when a new release comes out your modifications will not be overwritten