You should be able to specify a valid image URL and have it shown on both toolbar and ribbon. This does not have to be an image in a document library, _layouts folder, or within SharePoint. It can be on the internet as well. For example, You can search and find one online, "http://cdn1.iconfinder.com/data/icons/humano2/16×16/actions/gnome-searchtool.png". Specify the entire string to the ImageUrl property of the action item in Custom Actions property of qListForm.
If it is shown the broken image in your toolbar, could you show us the configured ImageUrl property? Also you can right click on the broken image and view it URL from the Properties dialog.