Starting in version 4.3, FFR supports loading resource packs. These are loaded with the "Import Resource Pack" button on the Presets tab.
A resource pack is a zip file containing various assets that the randomizer will load into the ROM. The currently supported assets are listed here. Every file is optional.
—Resource Pack Documentation
However, one needn't work entirely from scratch, a number of talented individuals over the years have made and shared their own assets with the community, many of which have been moved onto the wiki for easy access. If you're looking for a particular set of player sprites, for example, the Sprite Packs page will cover that.
In order to get started with creating your own resource pack, you'll want to read up on the documentation outlining how they work, though the main things to know are that you're combining assets into a .zip file and making sure everything you want in there is named correctly. You may also find pre-made asset packs below or on other pages of the wiki.
Either way, once you understand what you are doing, browse the index of assets below. For images, you may need to view them in a new tab in order to get a true-size version of the image. Additionally, some assets are pre-formatted for direct insertion into the game, while others are templates meant to be used as a guide-line for creating your own.
In addition to the files used for Resource Packs, there are several other files that may be uploaded or used with the randomizer to alter play experience.
So, after all of this, have you had a bolt of inspiration and want to contribute to the above? You can! Follow the formatting used on the above pages (and special instructions if those are included) and use the power of wiki magic to contribute to the community pool!