Images: if you're good at taking photos, upload them here. If you're more into graphics, you can help other users by attending to their requests. Or you have a collection of images that complies with the licenses in effect on hortipedia and you want to upload it.
But there are many more things you can do to help hortipedia grow:
Translations of help pages, images infos, etc.
Legal advice on copyright
Busy bees that browse existing files to see if categorisation and file info are correctly filled out.
On File requests you can see what files have been requested and make a request yourself.
So far, the tutorial and help pages are only in German (but we're working on it). If you want to know how the MediaWiki software works you may want to visit their online help.
The file factory (german) helps to enhance and customize files. Also, you can request the renaming of files that don't comply with the naming conventions or have typing errors in their title.
Online dictionary English - German - English
The online dictionary LEO can be very helpful to read the german help pages. There is a really nifty feature for various browsers to help with translations.