Assuming you're using Thunar, perhaps if you "Configure custom actions" via the "Edit" menu, then icons will be more file specific rather than generic. For instance, if nothing is set to open picture files -- jpg, png, etc. -- then likely you'll get a generic icon rather than a smaller view of the photo. I use geeqie to open photos, so within Thunar under custom actions for photo files the command to open them is set at "geeqie %f".
Or maybe you need to install some more icon themes. A quick search in Gslapt for "icon" will give various packages to install that might make things in your window-manager look more pleasing to the eye.