Basics

Using graphics: insert image, apply transformations, and advanced stuff.
Floating objects: to place and number floating objects like figures, tables, paragraphs, etc.
Combinations: adjusted images placement in rows and columns

Build your graphics

MetaPost: the graphical programming language
MetaFun - MetaPost in ConTeXt: ConTeXt graphical programming language, extension of MetaPost.
Sharing graphics: Export MetaPost drawings as PDF, import MetaPost drawings into MS Office, LibreOffice, etc.
The New (2020) graph module: with its dedicated manual "Drawing Statistical Charts" and a short presentation.
The basic graph module: Data plotting module, and some alternatives: TikZ (2016), Gnuplot (2012), Pgfplot (2009).
Flow Charts
Scaling Rotating Mirroring Clipping: Old Content (2010)
Gantt charts
Art and Artists

Layers: ConTeXt's mechanism for absolute positioning and advanced techniques like background / foreground, switch on / off.
Overlays: ConTeXt's mechanism to stack local snippets.

Including pages from PDF documents
Image database with XML
Image size for their placement: if you’re using ConTeXt for complex layout tasks, you may need to calculate with image sizes. Often it makes sense to use Lua functions.
Code to play with for Drop shadows
Example photo page layout (Old Content)
Animation, page mainly dealing with JavaScript animations.
Speech bubble
Even-odd rule in Metafun
variable color, example of Metapost / Metafun syntax with arrays of colors, strings, paths, pens and other variables.
Pascal's Triangle
textext positioning
Barcodes
Apply shading to text