Wiki has a built-in feature for uploading files which can be accessed from Special:Upload. This should be used ONLY for Wiki content, i.e. screenshots for articles.
There are also free file hosting services available on the Internet. Whether you want to share something big to make it available for testing or upload a screenshot for the forum, you can use one of these file hosting services. Below is a list of commonly used file hosting services. Or find one on Goggle, there are hundreds of them. There is also a topic on the forum describing how to use some of these services: How to post pictures, images and screenshots on forum (may be a little outdated).
NOTE: Beware of possible copyright issues arising from sharing/copying content.