JimmyCMPIT is certainly right the easiest solution for most people is to use the Slide Show screensaver. However for those slightly more adventurous another possible free option is Apple used to offer a tool called QuickTime Quartz Composer as part of Xcode, I believe this was actually used by Apple themselves to produce some of the included screensavers.
A long time ago I myself used it to produce a corporate screensaver module consisting of the company logo rotating in 3D.
It seems the last version issued by Apple was alongside Xcode 7.2. You need an Apple developer account to download it but a free developer account is sufficient. See Graphics Tools for Xcode 7.2.dmg and https://developer.apple.com/library/content/documentation/GraphicsImaging/Concep tual/QuartzComposerUserGuide/qc_intro/qc…
To a broad extent it is a drag and drop interface not a source code writing interface. I would still classify it as being for advanced users though.
Note: QuickTime Quartz Composer results can be simply dropped in to the screensaver folder and will then be recognised as screensavers.