frankly it should have 3 options to set the icon. if you don't upload an icon it should be an attachment. and it should also be possible to add an icon from an image url to reduce disk space and make it more user friendly instead of first downloading the image only to reupload it again if the image is already available on the web. ipb downloads has this feature and that's one of the major reasons i'm not considering of switching to xenforo. I would buy xenforo resource manager immediately then.
there should also be an option to set an default icon for every category, if it is not yet possible haven't tried it on the demo.
or there is also a possibility to grab the first image in the description and make that the resource icon.
the current icon system that i tried on the demo has this problem that you basically need to upload an image 2 times one time as attachment and one time as icon if you don't want the resource index page swarmed with the same default icon. so giving an option that an uploader can choose the icon from their attachments like O.P. requested is very much needed. The rest is just wishful thinking of mine
And probably the last request i have for icons is that people can choose from icons they previously uploaded so they don't have to upload their signature icon over and over again which is probably more important than icons from image attachments. But i do believe someone else requested this already. however this can be done if you make it so that attachments are reusable like with ipb. i believe there is an addon for it though.
anyways just wanted to say that i like this script but as a veteran who used ipb a long time i notice some minor things i really miss in this script which is understandable for something that is fairly new. but this has been on my mind a lot since the latest resource manager 1.1.0 is available for testing with this awesome new resource icon feature