Assuming you are using SharePoint 2013 (or newer) or O365, I'd suggest to create a custom link list which can be placed at the Document Library's (view) page. The list should be based on Promoted Links app which is available out-of-the-box.
On your SharePoint site go to Site Contents > add an app > Promoted Links. Name as you like. Create new list items to your Promoted Links -list and remember to specify the Background Image Location with the URL to the picture you wish to have for your link.
Once you have your links ready, on your SharePoint site go to Site Contents > Document Library you'd like to place the links > Edit page. On the web part zone, click Add a Web Part and choose your Promoted Links -list. You can choose whether to show/hide the list's title from the web part properties.
A working example:
- Promoted links come with a fixed size of 150x150px
- Promoted links do not have line breaks meaning if you have 8 links, they'll all be in the same row. A workaround for this is to have two or more Promoted Link -lists placed above the Document Library's view page.
The limitations can be altered with frontent customization, but that'll require some extra work.